LUN Migrator can be used to migrate smaller LUN to a larger LUN, it's a one to one relationship though. You can't migrate multiple small LUNs onto one big one.
I recently completed a storage migration from one array to another. I understand that your question is within one array, but what I am about to say may be relevant to you.
Are your partitions properly aligned? If you don't know what I am talking about, then they probably are not. I did a host-side copy of data from the old LUN to the new. I did this because my predecessor did not know what alignment offset is, and all the file systems were mis-aligned. Windows 2008 will align them for you, but if you've got any older operating systems or UNIX/Linux servers, alignment offset is a manual task.
Long story short, if you are moving data around, it may be worth checking into. Create a new LUN, present it to your host, align the partition properly, and copy the data manually. No, it's not elegant, and it's not very much fun, but if you're getting lots of disk crossings, it'll improve efficiency and performance.
If you right-click on a LUN, one of the selections is LUN Migration. This allows you to move a LUN to a different and potentially larger LUN within the same array. This is a standard part of flare and there is more information in HELP.
You must be aware that your host operating system may have problems seeing the extra space, so you would need to investigate that side of the equation.
There is also something called PowerPath Migrator - I don't know anyting about that, but I've heard that it can be used to move LUNs around.
If you are planning on changing the disk alignment with OS tools then you would assign the new LUN to the server, set the alignment with the OS tool (i.e.: older Windows would use Diskpart or Diskpar) then use a host based migration tool to move the data from the old LUN to the new LUN. EMC offers Open Migrator/LM (versions for Windows, Linux and Unix available) to perform online migration. if alignment is not a factor then the easiest way to move the data within the array is the LUN Migration option previously discussed.
Are you saying that you want to move LUN 100 from a RAID Group built on 146 GB physical disks to a RAID Group that is built on 450 GB disks? In that case you create a new LUN on the new RAID Group and use 'LUN Migrator" as described above. The old (source) LUN number takes over the new (target) LUN Number; i.e. - the end result is that the data is moved and the new 'space" becomes LUN 100. It's included in NaviSphere and is free.
If you get "holes" on your physical disks, then you can "defrag" them within NaviSphere. I think that you click on the RAID Group and look for the 'defrag" or "compress" button. It runs in the background. also free.
For Raid Group defrag, right-click on the Raid Group name and select Properties, then go to the right tab (Partitions) - the defrag button is at the bottom - be aware that raid group defrag and take a long time depending on the size of the raid group and the types of disks used. A very large raid group using 1Tb+ SATA disks can take weeks to defrag. Once started you can not stop a defrag. It will restart if you lose power or the SP reboots.