This article gave me a reasonable answer. https://www.dell.com/support/article/us/en/19/sln129581/understanding-hard-drive-types-raid-and-raid-controllers-on-dell-poweredge-and-blade-chassis-servers?lang=en "Dell PERC (PowerEdge RAID Controller) and other controllers can support a variety of hard drive types. There are four main types used in Dell 9th generation servers and up. There are specific configuration limitations and the specifics should be checked for the type of controller used. In addition, the types cannot be mixed in the same RAID set." Is it possible to mix different type sizes? like 300 GB and 600 GB HDD's?

I am assuming all these drives connected to one of the PERC controller (H730/H740 etc). As long as new drive has same or large capacity with same protocol based (if existing is SAS and new drive is SAS OR if existing is SATA and new drive is SATA) specs can be used for replacement purpose. Option such as

1) assigning all the new drives as dedicated hot-spares to this RAID group. And removing 1 drive, allowing rebuilding operation to complete by using one of the hot-spare. Repeating the process till the replacement of all the drives with new set of drives.

2) Performing replace member operation from existing drive to new drive. Allowing operation to complete one by one till the end of last remaining drive with new drive.