Can you replace a 20-yr old Unico system with a Spacepak

It all depends both are high velocity air handlers and either may be used in that type of system but depend on the material used for your duct system and weather it is tight and not leaking air . 20 years ago even w hi velocity system guys where really not sealing system all that great . Now a days all joints are and should be sealed w a gasket tape or mastic . I personally prefer to do high velocity systems using spiral round duct work ,proper gauge metal , no length long snap seam and non adjustable 90 and tees so much if not any air leakage . Also I would image that your duct insulation isn’t up stuff . I usually use a r 8 foil faced wrap 20 years ago we where using r 4 big difference . On small note 20 years ago the the accepted route was 6 registers per ton now a days it’s a mim of 8 per ton . So from my experiences replacing a 20 years unico or spacepak isn t just a swap like all things in life it will require a little bit more work then slip it in also note that your going from r 22 to 410 I would suggest a new line set which will be the cheapest thing to do on the whole job . Personally I install unico it just always seems as I rarely see a great high velocity installs done by the book but I see more crappy hacked in space pak jobs then I ever see hacked unico ,I think it’s basically the supply house that sell them and the type of contractors that frequent them .both are expensive but either one installed corrrectly should be as quite as a standard system when installed correctly and installation details are followed . Beware not all sheep’s are sheep’s and make sure that it not gonna just be a quick swap out and if registers need be added they are added not after there’s issues . And if using a ecm motor on indoor unit install a surge protector it cheap insurance Peace and good luck clammy