Someone just offered me a 1984 pair of Heresy I's for my RF-52/RC-52/RW-10D..  Are the sonic characteristics that dissimilar, can I get a conversion kit ? 

I was thinking of holding back on the RW-10D, really cool piece of work. 

Maybe you can do a side by side eval......

not quite an apples to apples comparison. You really need to do some side by side listening.

I'm a huge fan of Heritage sound, but not everyone is. Holding back the sub for your future use is an idea that has merit.

One thing, twenty years is the lifespan on crossover capacitors. The Heresy's will need to be recapped to return to factory spec and sound their best.  That's not an expensive proposition and easily accomplished. You could check with BEC on this forum to get more info on the cost. He's the most reasonable option I know to handle this. He can support DIY with parts or a turn key job if desired. 

Another thought on relative value, I don't know what the used market would be on your system, but the used Heresy's would be between $300 - $400 on the used market.