Compensation for not being able to stow our tub/shower units in the attic? (A real trick in this house, and we've considered a replacement!)
Depending on whether you want a "walk-in" or not, the changeover needn't be too traumatic.
Your old unit can be destructively dismantled. Then a new tub, and a "three-piece" surround, can be put in its place. Doing it carefully, and properly, could take a couple of weeks.
But first, you have to decide what features you want, and whether any compromise is needed. And then, how much work you may choose to do yourself, versus paying exorbitant sums for quality craftsmanship.