Is this a tiled shower? A partial tear-out is one way, but once you've gone that far (the pan is the hardest and most complicated), you may just end up tearing the whole thing out. To install a liner, you must tear out the floor and curb and up the walls at least 6" or so, probably a little more. Anything else you do is a patch that has no guarantee of operating properly.
The floor and wall could have some major rot issues if it has been leaking internally for awhile. You won't know until your tear it up.
You probably should read the condo by-laws very carefully, some of this may be covered by the association, and they will probably have some say in what's done, especially if it affects the structure.
For good help on showers, repairs, and replacement, suggest you check out www.johnbridge.com.