The first bass I got when I started out playing was a Squire Precision Bass. Definitely a good starter bass but it has some cheap hardware on it. The wood is solid enough though to keep it around and use a backup to my Carvin IC5WP.
I decided to upgrade it and ordered some new parts. Although it was probably overkill in terms of quality, I replaced the stock bridge with a Leo Quan Badass II bridge, replaced the stock pickups with Seymour Duncan Quarter Pound pickups and replaced the stock nut with a brass nut. While they claimed to be direct replacements for a Fender p-bass, it was a bit tight for the pickups and the nut was too long. After some grinding and filing of the nut, it now blends well with the neck. The wiring of the pickups to the potentiometers left a lot to be desired so maybe I'll eventually get around to re-wiring them. A new jack might also be a good idea.
While the action is still high, the sustain has really improved. The pickups definitely have give the bass a better (and wider) tone. So while it's not perfectly setup, I'm happy with the results so far.
I have some new effects pedals and a small combo on the way so I'm looking forward to uses both basses with them now.