Approx.How much would it cost to get this done to my bicycle?

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I have a 1995 Giant MTB still rides smooth and I ride it daily.My first question is should I upgrade or invest into a new bicycle?I love the bike I have now and couldn't possibly afford a bike like this again.Approx.how much would it be for the bicycle shop to change gear shifters/brake shifters,Change from a 21 speed up to a 24-27 speed,and change out the shifters?Including parts/labor please any help would be appreciated.One more thing is there any possible way I could learn how to tru rims?If so please let me know that as well!Thanks