On a original setup ,yes if the base is in place as the antenna screws into the base then the chrome bezel goes on then the antenna nut! They did make after market ones back in the day that went into the fender hole and clamped down! They were not the best not sure if anything like this is still available!
So you can replace the mast, but not the base. If you need to replace the base, you need to get in there.
Seems like I remember doing mine, base and all, with the fender on. It is factory. Maybe if you pull the vent screen on the cowl?
That's up to you. I never pulled my fender. You can always try before you decide. Remove the cowl screen and reach in.