If you put a Partner R7 processor in your old cabinet, and then reused your 206 cards, at least your 1st 5 lines would have caller ID; or if you went with just 1 addl 308 card, you would have 8 lines w/ caller ID (which is what you have now) and then you could save your remaining 206 cards for spares if/when the old ones start to give out. Its actually a pretty easy swap - I do it literally every week. And then...since you have a new system, there is a PCMCIA-based voice mail you could add to the processor when you are ready for on-site voice mail. It is actually a nice upgrade path that lets you make changes a little bit at a time.
The only thing to watch is in order to put the R7 in the old cabinet, you will have to take a pair of plyers and snap the tab off the middle slot on the bottom of the "box" all the cards fit in - it interferes with the changing of backup batteries on the new processor.