for a quick test, i think you could install the 5.3 agent and leave your connection sever as it is. but, certainly not for long term production use -
I'm not a qualified expert on what can be used with what, but I'm pretty sure for what we are trying here you could just try upgrading the agent for now. lets solve that, and then loop back around to solve the rest of the infrastructure.
if of course once you do that then you can no longer connect to the desktop, then maybe Im wrong and it all does need upgrading.