How best to upgrade

I would suggest:
Determine your budget
Prioritise what you want
Understand your room/listening position
Consider appropriate room treatment
Audition appropriate speakers
Find appropriate matching amp for speakers
Find appropriate matching pre/lne controller for amp
Find appropriate sources
Find appropriate rack/stand for gear
Choose appropriate cables
Set up appropriately
Reconsider appropriate room treatment

Good luck!

Not sure where to start? Listen more...

Seriously, consider all that Unsound listed but on top of that just listen even more. After you do a bunch of that are there just certain CDs where you think the sound is "just off" or after listening more and more and more is there something in general that you think ain't quite what you want today. And that's another point - remember that your hearing has probably changed a little over those years and you might not appreciate the same overall sound you once did. If you think that's on target then probably concentrate on speaker alternatives (or room improvement) - or maybe just listen more to start to (re-)catch those subtleties you used to really like about listening to your stuff.

Reposition your speakers, listen for a few weeks, then reposition them again, listen again and so on and on....... until you get it like you want it.