I have a 9 year old 9spd Ultegra Triple and the front der is getting cranky. It shifts fine from the little ring to a bigger ring, but won't downshift well. I can push it down by hand, (or push it with my foot if I unclip but it doesn't shift well.

I think that this is from getting gunked up. In the past when this happened I cleaned it well, or even took it off and soaked it in some carb cleaner and it would be better for a while. I wonder if it's just old and the spring is weak.

Am I better off just replacing it? Will a 10spd front der work fine with my 9 speed setup? (In case I want to upgrade to 10spd someday)

Thanks

Could be the spring but the more likely suspect is cable friction. How long has it been since you replaced the cables and housings?

I did the cables a few months ago, but not the housing. Think I should try that?

It sounds like they're old, so yes.

Disconnect the cable at the derailleur and move the cage by hand. If it moves inward freely when you let it go, the problem is in the housing. If it sticks or is reluctant to move even without the cable, it's the spring or pivots and the derailleur should be replaced.

Also check/clean your cable guide under the BB shell.