Making normal repairs, of the type that would have been done in its service days, would not decrease the value as long as the parts used were properly made. Replacing any major parts, like the cylinder, grips, barrel, hammer or loading lever would decrease the value unless original parts were used (it is still possible to get some original parts for those) and the ones on the gun are broken or totally ruined (barrel bent or crushed, for example).
The gun does look as if it might have been reblued at some time, but it is not possible to tell without better pics. If the condition is original, it is outstanding.
Preservation would be best done using a good gun grease (Rig is one) or oil, a good cardboard box and non-acid paper, also oil soaked.
Jim