i have a USP .45 that is date coded 93.  i bought it used from a friend for $500.  ive had it about two years, and im starting to think it may need a bit of work done to it.  everything feels kind of loose, somewhat sloppy.  also at times when the slide is locked back, if i insert a mag it will auomatically slide forward.  

id just like to give it a workover and make it better.  its a good shooter but i dont think its at its best.  i have a USP compact in 9mm that is almost new that i use as a reference point.  also, it has a fiber optic front site, i was thinking of changing it to traditional night sights. dumb idea?  

You'd be best suited to go back to the factory, IMO. You seem to have a laundry list for problems & H&K warranties their gun for lifetime, AFAIK. OTOH, using a private smith means you have to first find someone who can do it, then you have the SEE if he can do it & then you have to decide if he can do it. Too risky, IMO. I'd do factory & keep the warranty intact.

My .o2

Factory is a good way to go unless you can find a factory certified "armorer" or smith in your area.  The HK Lifetime warranty is for the first owner of the gun.  I don't know if there is a way of proving that the other guy was the first owner.  If he never sent the card in, then you might have a shot?  

I'd call HK in VA for a RM number and then send it in to them.

I bought a used USP45C on the EE here that malfunctioned and had a sear part fall out of the magwell into my hand.  Turned out the previous owner was a home gunsmith of sorts.............

Any way - HK went through the entire pistol and replaced everything that wasn't up to factory specs, test fired it, and returned it to me free of charge.  Outstanding customer service.

I too would recommend sending it back to H&K.  They are great guns but they have one draw back, 6 million pieces and only a few qualified people who know how to work on them.  The problem you have with inserting a new mag on an empty weapon sounds normal and that will happen from time to time.  It happens when the new magazine is insterted too hard and at a certain angle releasing the slide release spring.  Good Luck!!