Thanks for the feedback so far. Not sure if @theacura @GuntherPelgrims check these posts anymore. But I'm still running into issues and not sure if you know off hand from my below description. Otherwise, no worries and I'll continue to search other forums.

Before switching SDDs, I downgraded all the way down to 2.5.1.

After I insert the new SDD, I get the following error "no bootable devices found". On my USB stick, I have both previous BIOS files and the Windows 10 update (downloaded from here: https://www.dell.com/support/home/en-us/drivers/osiso/recoverytool/wt64a). Not sure if having all files on the same USB stick would be an issue?

I also search the BIOS screen and notice the new SDD does not appear (it's not recognized under System Information).

I'm currently searching other forums because this sounds like a different issue altogether than the original BitLocker issue.

I have "successfully" put my old SDD back in and received the same BitLocker prompts I'm trying to resolve. So at least I'm not worse? Lol

Thanks again to this community. At this point, I'm holding out hope it's just a setting somewhere I have enabled/disabled which prevents me from progressing forward with this latest solution.

@x60643 You don’t currently have a bootable device because you need to install Windows on your new SDD. I forgot to mention that I was unable to install a fresh copy of windows 10 using the Dell OS recovery tool. 

Google “Windows 10 usb install” and go to the Microsoft page to download the Windows 10 Home edition install from your USB. It will format anything on your USB so save anything you need elsewhere. Once installed, it will boot into a fresh copy of windows on your new SDD.

After that, install all of the windows update once you are back into windows on your new SDD. Once all updates have been installed, put back your old SDD.

Hope this helps and got my fingers crossed for you!

Great to see at least some hope here. My new hard disk has just arrived.

@isama @GuntherPelgrims @theacura :

When reinstalling Windows on the new disk, have you specified anything about Bitlocker? Did you have to connect to any Microsoft account? 
Did you disable UEFI capsule firmware updates in BIOS?

Hi @fabian.b 

I didn't start it yet (disc not arrived) but theoretically first 2 options absolutely irrelevant, new windows goal is just to bring correct BIOS update.

Third one is important - for some guys ,when "UEFI capsule firmware updates" is not disabled- it automatically updating BIOS to 2.16 before start and  "bitlocker recovery mode" remains active.

Thank you!

An additional question: did you also install optional windows updates?

It worked!

Thank you @GuntherPelgrims for providing the solution!

Btw. For me, system stayed at BIOS 2.5.1 and there was no upgrade.

And a maybe very important hint for the still desperate ones:

After recovery, I chased my Bitlocker key and found it:
I have an account at Microsoft just for accessing Sharepoint and few other tools, but with a custom email address. With this email address, I could not login to the pages Microsoft was hinting (e.g. onedrive.com/recoverykey or account.microsoft.com (Microsoft account does not exist)

BUT

I can login to:
myaccount.microsoft.com/device-list  
--> my bitlocker key is stored there!
So, try the last URL with your email addresses (also if not belonging to microsoft, in my case it was with posteo.de)  and you might be lucky even without ordering a new SSD!

@nomad 

I had UEFI capsule firmware updates deactivated. I used MS Media Creation Tool. 

Why are you manually installing 2.10? Have you tried replacing disk before when it was still at 2.5.1?

I'm still struggling to conquer it.

Attempt#1&2

• Downgraded BIOS to 2.5.1;
• Tried to install both images created by Dell OS Recovery Tool/MS Media Creation Tool; Installed all Windows updates along with BIOS 2.10. Windows Update indicates that the system is up date date;
• Put the old disk back, unfortunately BitLocker key prompt is still there;

Attempt#3&4

• Downgraded BIOS to 2.5.1;
• Prior OS installation checked option - UEFI capsule firmware updates;
• Installed both images created by Dell OS Recovery Tool/MS Media Creation Tool. Installed all Windows updates without Firmware (due above statement); Windows Update indicates that the system is up date date;
• Manually installed 2.10 using USB drive via F12 Menu;
• Put the old disk back, unfortunately BitLocker key prompt is still there;

@GuntherPelgrims @theacura @Crasher9998 @fabian.b 4 persons confirmed that above workarounds have worked for them. You guys have succeeded and I'm wondering what I do differently so I could not?!

@x60643 It seems what working for others, is not working out for us. Is your laptop still under Dell warranty, if yes, have you tried to contact Dell?

No, no longer under warranty. Yes, I've contacted Dell but the folks I talked to were nowhere as supportive or knowledgeable than the folks on this thread. I paid for the support appointment (since I'm not under warranty). After the call, they ended up reimbursing me the charges since they couldn't resolve it.

@nomad I’m still having issues as well. Will try to document what I did, but not sure where to properly get the Windows updates. System started to do automatic updates once I installed the new SDD and W10. Ended up going to the Dell site and downloading all except the latest BIOS. Switched SDD and still get asked for BL.

@fabian.b I quickly made fresh installation on 2.5.1 with deactivated UEFI capsule firmware updates. Put the old disk back, still no lack. However, I haven't installed Windows updates yet.

Did you install all Windows updates prior putting the old disk? I thought it is a mandatory part of the game as we needed BIOS from Microsoft. If your old disk booted without installing updates on the new disk, this workaround is not working in my case.

Unfortunately myaccount.microsoft.com/device-list  couldn't help sometime (for me):

"You can't sign in here with a personal account. Use your work or school account instead."