Well, the idea when you split is:
You get a FE (front end),
and then you get a BE (back end data file).
Now, you place the compiled accDE FE on each work station.
Since they are (as you noted) using the accDE FE, then you are free to work away on the next great version.
However, the BE? Yes, if users are working and using the FE, it will lock out the BE from design (table changes).
Two solutions:
Schedule a time or after work, you have to make those changes to the BE (data only file).
Or,
Make a copy of the BE, copy local, or to another shared folder. That way you can make changes to that BE, but you need to "keep notes" or keep track of what you did. Since you eventually still have to open the BE production BE, and make the changes to the BE data file.
Of course, you are free to make changes to the FE part - since users each have a copy, and in most cases have a compiled accDE copy. So, your 100% free to make changes to the FE that's on your development computer.
However, the BE? No, you as a general rule can't make changes to the tables and table desings while it is in use. You have to schedule this time off work hours, or as noted, work on a copy of the BE, but then you need some notes or word doc in which you write down what you changed, and you have to remember/make those same change to the production BE when you have that off-hours time).
Now, if you can't for some reason make a change to the FE?
Well, this sounds lame, but re-boot your computer - you might have a stray copy of access running. (I susppose you could also exit access, and check the task manager - see if any stray copies of Access are running).
Now, of course if the FE is not actually on your computer - (very slow and very hard to work on the FE if it is NOT on your computer). And it also very possbile that someone else has the FE open - which again is a good reason why you want to edit local on your computer.
So, EVEN if the development, or the accDB on your computer is linked and connected to the production (live) data BE? You are still 100% free and should be able to change forms + code at will.
Now, of course any change to the tables structure(s) has to be done on the BE, and not the FE. And as noted, that you can't change at all while users are working away.
Regards,
Albert D. Kallal (Access MVP 2003-2017)
Edmonton, Alberta Canada