What version of ePO are you using? are you removing all workstations or just some?
Just some (80 old computers),
the other are the new computers or recent computers.
You can do it in many ways so it will be down to personal choice and your environment.
There is a server task already for inactive agents which is disabled by default and if enabled will remove agents that haven't communicated in the last month. You could change this query to smaller time frame to coincide with you PC upgrade, but this may remove additional machines that you may not want to remove so some additional editing maybe required.
You could create a query based on unique information to do with the old machines, this again will depend on you environment, example most of my clients use some for of numerical system in the PC Name eg WORK10001, WORK10002 so the new machines will end up with the higher number. Create a query to collect all machines with a number less than the first new one and set a delete system server task against this query.
You could also create a tag group based on again some unique information for the old machines and then sort these into a system tree group and then delete them this way.
More ways are possible but you just need to find one that will work for you
I'd use the inactive agent maintenance task - this is exactly what it's designed for
If you leave the "remove agents" box unselected, then if a machine that is still alive is removed, the agent will not be - so it will reappear in ePO the next time it communicates. The genuinely dead machines will never try and communicate again and so will never reappear.