Unico Outdoor Unit Replacement before R410 change

[dutchglider]

I (homeowner) have a Unico system running with a 17 year old outdoor unit (straight cooling-4Ton).
The outdoor unit is probably near end of life. I am interested in replacing outdoor unit this year because of
imminent industry changeover to R410A systems next year.
Points to consider:
1) Replacing outdoor unit with new R-22 unit; more than $xK,
probably <$xK with new line set.
2) Replacing outdoor unit with new R-410A unit adds about
>$xK as the indoor evaporator coil needs swapped out
and is quite expensive.
Risk is that indoor coil breaks/leaks within a few years,
then I would have to buy a new indoor coil compatible with
R-22 (if available) but then this new coil would be obsolete when my R-22 outdoor unit reaches end of life.
What are my chances of having indoor unit catastrophically
fail before a new R-22 outdoor unit reaches end of life? Low (<20%), Med (<70%), High (>70%)
The indoor coil is probably 25+ years old.

Yet another alternative is to install a chilled water indoor coil and get the unichiller. Water won't change like R-22 -> R410 -> to whatever plus I could use that fifth ton of capacity on the hottest days without coil freezeup (I think).

If I replace indoor coil and outdoor unit, should I consider
heatpump for the shoulder seasons. I have a hot water boiler to feed a heating coil.
Your thoughts on evaluating the costs/risks?
Thanks!
Dutchglider

[catmanacman]

at25years old it is past the average life expectancy for a indoor unit i would replace both outdoor and indoor unit and probably go with a heat pump for the mild part of the winter season then when it gets real cold switch to hot water

[beenthere]

Any new outdoor unit you put on that old indoor unit, will lose lots of its capacity.
May not cool near as well as the old condenser does.

Heat pump, would probably pay for itself fairly quickly. And be the better way to go.