Generally with gut the bridge and nut slots need to be widened somewhat. Also you usually will want to raise your action some. If your bridge has adjusters this is no problem. Likewise, if you go back to steel you can lower the bridge again. I haven't experienced any real problems with putting steel back on slightly too wide or deep slots, they should not be too deep however, If the relative heights of the strings are the same you shouldn't notice much difference. Another option when going back to steel is to have your bridge reworked, i.e. sanded down and having new slots made, raising everything up a little again with the adjusters. A third option, used sometimes by string junkies like myself, is to fill a slot with Super Glue Gel, use an instant hardener like Zip Kicker, file it smooth, and the make a new slot with a small rounded file. A fourth option is that you could have another bridge made for a gut setup.
To give a simpler answer
I've had bridges where I could go back and forth between guts and Spirocores doing nothing but raising and lowering the adjusters a bit, but it can depend on how particular you are.
>> Perhaps the D string can be the trickiest. Gut D's tend to be quite thick proportionally and sometimes require a deeper/wider slot. Better to leave the D a little high if you think you may go back to steel, so it won't be too low then. It doesn't hurt to keep a gut D a little high anyway, for tone and tension.<<
