I have 2 decent, low-cost "working man's" basses. I've been thinking of an upgrade. Same specs, basically a nicer version of a bass I already have. I want to separate inspiration from GAS.
IMO there are 4 upgrades, 1- to sound, 2 - to feel, 3 - a better bass, 4 - just because you can.
1 - New pickups or a preamp, or an outboard preamp pedal.
2 - Professional set up, fret level and new set of strings.
3 - A more expensive bass.
4 - Do what you like. It's you money to waste.
I have upgraded the sound of an old P bass with new pickups, didn't cost too much and made a huge improvement. Then added a J pickup, cost more and didn't add much as I'd hoped. Then I put a preamp in, which cost a lot more and gained me very little.
When I buy a new bass, the first thing I do is to get it properly set up and the frets checked and levelled if they need it. Even the best basses can come back feeling like a million dollars.
So my experience and conclusion is to buy the bass that sounds like you want it to sound and forget any upgrading and always get your bass professionally set up. The upgrade you're looking for is the next level of bass. Choose well and you'll find a night and day improvement that can't be achieved buy just buying more parts for your existing bass.
The best basses I've owned have been a Wal, Lakland, Lull and Sadowsky. There's no going back after playing these instruments.