As I have some experience in this from a diferent direction I'll relate mine. I went from wide to narrow, an O-16 to an F-30 Guild. The nut width was only an 1/8" of an inch diference but that diference was telling.

First think in terms of muscle memory. (Rick has spent hours and and hours over the past 30 years making a living with instruments. He may not realise how aware he is of muscle memory.) About the time you get used to an instrument you can play by "Radar" almost. You develop an economy of motion that suits you and the instrument you play. You don't have to watch what you do. ( Doc Watson, Jose Feliciano, Riley Puckett) Then you screw up the mix by changing to a diferent instrument and the whole process begins again.

Changing from wide to narrow is easier than the return. Its somewhat easier to compress than to stretch a muscle. Its the stretching that strains the muscle and forces you to rest awhile. I my case the reconnective surgery means I can open my fingers beyond 60% so the beautiful Martin sits in the case waiting for a friend(s) to stop by with out a guyitar to play while they visit. I play the Guild or a narrower still banjo.

The depth of the neck creates its own unique set of problems. Deeper necks mean more reach is required and the fingers need to go further around the finger board to create chords. This makes it harder to hit the chord you know and you have t stretch those fingers to accommodate. Suddenly the Barre Chords are needed when you didn't use them before. Try any number of handmade necks from the same company on the same model and you'll be surprised at how different the feel is from one D-28 to another.

Rick, my bet is you are battling muscle memory and probably repetitive motion illness and maybe some arthiritis.

Don