I was in the same position.
I got a pac lite upper for a couple of reasons.
Cheaper than a new gun.
Same mags.
Nice rail mount - the pac lite, a Burris FastFire or similar small red dot, and a Sparrow is a tac driving, whisper quiet package.
I'd done quite a bit of work on my Ruger lower - new trigger, hammer and sear for a great trigger pull, and an extended slide release, so I wanted to keep that set up.