Who knows how much longer the power pac of that 20 year old EAC is going to last but if you're satisfied with the service it's given you keep using it.

The power supply was replaced a few years ago, along with the mesh filters, so I guess it will stay!

look in the blower compartment to see how much dirt has bypassed the filter. If it is clean the filter works.

The power pack can be replaced along with other parts.
20+ years and a new furnace, I would consider purchasing a new one. However, the contact boards, and associated other parts can be replaced if your handy, along with cell contact board replacements, pre-filters, etc. There is nothing new and different about a brand new one and the old generation.
Here is link and a company that provides great service.

Here is a link that might be useful: Bel Air

We ended up keeping it with the new Carrier furnace.

I agree, Bel Air is a great place to do business with.
I did business with the (I believe)current owners father, when he worked out of the attic of his house in Westwood MA.