I never do those kinds of things without a shirt on, so I don't see a problem.

Replace the scapular when it is worn out, using whatever criteria you have for other clothing items.  Some people wait until it is virtually falling apart.  Others replace it at its first fray.  There is no absolute standard on this.  The decision is up to the individual.

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The only 'rule' is you can't throw them away when they're worn
out, but you should either bury them as if they were deceased,
or burn them.  Some chapels have a collection jar where people
put their worn out sacramentals, and the priest burns them all
every so often.

Then when you get a new brown scapular, you don't need to have
it blessed, because you have been invested by a priest (that is,
you should be invested so this works like this in the future).  
After you're invested (the priest puts the scapular over your
head and says some prayers), then from then on, all you have
to do is put on a new, unblessed brown scapular and it becomes
a blessed scapular because of the fact that you have put it on.
This is part of the reason that a "scapular medal" doesn't have
the same effect as a cloth scapular.