I would not suggest replacing the tub with a shower only unit as that does affect resale. Any buyers with children will definitely want a tub. Standard upgrading to a new tub and or/tub surround with re-glazing however is a good idea.
If you are talking about converting to just a shower, and are thinking of resale in the future, you should see what comparable properties are selling best to see if it is a wise decision. If you are simply updating the bathroom that is always a plus.