Alternator interchangeability- 2003 Forester using an earlier alternator?

[Stevo F]

I replaced the alternator on my 2003 Forester a few months ago with an OEM new alternator. Now I'm finding, with a voltmeter, that either it takes a few seconds to art charging or not at (it takes several starts for battery voltage to go from 12 to 14 volts). I installed a crappy Duralast alternator and it pushes 13.5- 14 volts all the time.

I'd rather use OEM than Duralast (it was originally on my Outback when I bought it and I replaced it with OEM on that car). I already have an unused Subaru remanufactured alternator that I had purchased for my '98 Legacy, but sold the car before I could install it. My question is whether I can use the older alternator on my Forester. I'm not that concerned that it's only an 85 amp vs. my current 90 Amps, but the electrical plug is a different shape, although it has two wires like on my 2003 Forester. I figure I could just wire in the 90's style plug, but are there are design differences between 1990's and 2000;'s Subaru alternators that would prohibit me from using the older alternator?

 

[nvu]

I went reverse, 2006 alternator in 98 impreza.  Swapped from green oval to black rectangle plugs and works fine. 

Though 13.5-14v at the battery is kinda of standard nowadays, it's okay for newer batteries.