If you're going to wipe everything and start from scratch, then just detach from the project, and re-attach when done.
If you're just updating things, then do the updates & continue crunching your existing cache when you've finished updating.

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Grant
Darwin NT

If I detach will this download the same tasks again as I'm going too completely wipe the HDDs for a fresh OS Installation.

Nope, you will get new WUs.
But when you detach, all your current unprocessed WUs will be released again, so people don't have to wait a month or 2 for them to time out before they get resent to another system.

Starting from scratch this way will get you new computer IDs
It is possible to make a copy of your current programme & data directories, and copy them back over the new installation of BOINC on the newly installed OS to keep your existing computer IDs and work caches, but it involves a lot more mucking around than just detaching & re-attaching to the project.

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Grant
Darwin NT

Can I ask. By detach do you mean select seti & select remove from the options on the left hand side.

Yep, Remove.
Set No new tasks, do an Update to report any finished but not yet reported work, then Remove to release all your current unprocessed work back to the project.

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Grant
Darwin NT

My first choice would have been to backup the data directory. Then restore it afterward.
Otherwise I would just abort the tasks from BOINC. That nearly immediately puts them in the queue to go back out.

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SETI@home classic workunits: 93,865 CPU time: 863,447 hours
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You could considering staggering your upgrade.

If it's just a version bump of the OS [Linux Mint] or a switch to another linux distro.
Upgrade your primary machine/s for example and then perform the upgrade on the rest of your machines.
And migrate the BOINC Data Directory across of each machine

If your also taking the opportunity to change the hardware I'd probably do a project detach of the machine.

finding a Second Sechedule window could be a problem though.