(Per Max’s NOV 2019 updated comment regarding not having to split the drives before replacing the HDD — posted in Answers so my post was more visible):
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So for a late 2014 27" iMac with a 1 TB HDD and 128 GB blade (currently running Mojave), I DO NOT NEED TO SPLIT the fusion drive before I replace the HDD with an SSD?
I am planning on formatting the new 2 TB SSD in AFPS format before I install it... and was hoping to fuse the drives because my blade is not large enough as the boot drive (with all the apps I currently have installed — currently as a fusion drive, I only have a few hundred MB of space left on my blade). I read on the comments section for the 2014 replacement kit (on ifixit) that someone successfully fused their SATA SSD with a blade (with both SSDs formatted APFS). I was hoping to fuse the SSDs after replacement to make sure I have sufficient space for all my apps — it's not in the budget to buy a new larger blade, besides the fact I was a little worried about replacing the blade successfully.
Can anyone tell me if I will be able to fuse two SSDs in Mojave or Catalina? If not, how would I make the SATA SSD my boot drive — just by formatting the drive with the new OS (i.e., will it automatically boot as my startup drive if Mojave or Catalina are installed on the SATA)?
Want to make sure I do this correctly so I don't have issues after replacing the HDD.