I was hoping someone more knowledgeable than me would have replied in the meantime, but I’ll try to explain my recollection of the status quo.
Updating (as in within the same /e/OS Android version, currently 10) an FP3+ bought from the https://esolutions.shop/ will not differ from updating an FP3+ that was switched from Fairphone to /e/OS by the user himself or herself. It’s an OTA update (through Settings > System Update) and your data will be carried over.
Now upgrading an FP3+ from /e/OS Android 10 to /e/OS Android 11 is a bit of a challenge for the /e/ developers right now. They are still working on making this upgrade available through OTA so it will also preserve your data (i.e. no “clean install” from scratch necessary). I am quite optimistic this will succeed, but even if it does not, I think there will be no difference in procedure between a “bought” and a “self-switched” Murena FP3+.
I recommend you follow @Manoj’s weekly updates, especially the “OS OTA Upgrade Testing” section: