Can I use my existing re-entry permit after renewing my visa in Thailand?

You don't have a new visa. You have a new "Extension of stay" for 1 year. If you want to leave the country you must have a new Re-Entry stamp.

Not worth risking it. Get another one.

When i apply yearly for my extension of stay, for which I have a multi-re entry permit, the re entry permit becomes void from the day i submit the application, although it has amonth still to run. If i want to leave the country while my paperwork is considered i have to purchase another Single re entry permit to cover the under consderation period. Then when the extension is approved a new Multi re entry permit with the same dates as the new extension. This obviously leads to overlapping dates on the permits, but only the most recent is recognised.

No. That re-entry permit was “tied-in” to your previous extension. You need to get a new re-entry permit to keep your new extension alive.

The re-entry stamp will have a valid until date on it, you can use it until that date I believe

I thought when you renew the visa for the next year, they void the old visa in your passport?

No it’s now in valid ! Cancelled when you renewed