A factory reset would not be required unless things get very muddled.
Although the following topic is about a FP2 the method remains valid in principle. I have not tested it: one of the problems when one wants to help people with internal storage on SD, is that ideally one needs to devote a phone + card to pure experimentation (I haven’t, yet).
Read through to the end of the topic.
I would add, that once you’ve removed the SD card it could be re-formatted for use as Portable storage. I would do the formatting on a PC. You might want to use the formatting tool provided by the SD Association.