Hypothetically speaking, if one owned an AN/PRC 90 survival radio, how is the upcoming PLB frequency change going to affect its validity as a survival/rescue tool. I realize it won't transmit on 406 for the beacon, but are the voice freq's still going to be monitored and will the SAR folks still use the homing beacon freq.???

Freqs used by the PRC-90 are 243.0 MHz and 282.8 MHz.
243.0 is an International Air Distress frequency and is what you get when the Guard channel is selected on a military radio. That freq isn't going away any time soon and is monitored by any military radio w/ Guard selected.
282.8 is UHF SAR COMMON is a discreet freq to take SAR related comms off the guard freq.

The most critical change is that it no longer serves as a distress alerting device in beacon mode. Distress alerting was a function of satellite monitoring, which is what is going away. So, don't expect it to get you rescued if you find yourself in distress.

Essentially, what you have now is a rather bulky two frequency VHF transceiver which takes an expensive battery (though there is a adapter to allow use of CR123 cells) Those frequencies are still in use by the military as emergency frequencies, but are not monitored or used for civilian use. You might get lucky transmitting a mayday on 243 MHz if there happens to be a military aircraft in the area who is guarding that frequency. Not something I'd want to bet my life on.

Anyway you look it it, from a practical perspective it's pretty much become a collectors item or paperweight.

Honestly, the darn thing was next to useless the one time I REALLY needed it. However, it gave me a warm fuzzy feeling the whole time it was there. (at least until it didn't work all that great)

The PDF had the ability to RDF the beacon, so I only transmitted when I could see an American aircraft. I finally got someone to respond when in sight of the good guys on the ground.