A small nick or mark is usually not a problem – you can usually print right over it without any noticeable flaws on the bottom of the part you’ve printed. From experience , even a large gash can be tolerated (whilst one is waiting for a new PEI sheet to arrive) – as long as you can shift the part around in Cura so that it avoids the defect. I’ve done this process a few times now, the sheet is considered a “consumable” and I’ve learned it’s wise to have one on-hand at all times…
Here’s a link to a replacement PEI sheet in the Lulzbot store:
And here’s the procedure for replacing the PEI sheet on the Lulzbot Mini. Not sure there’s an equivalent for the TAZ, but it’s the same with the possible exception of the process for unplugging the connectors for the heater.
Also, if you TAZ is really, really new, I think it’s possible that it has the new modular bed – in which case, the heater and the aluminum plate to which it’s attached stays, and all you have to do is lift the glass off once you’ve removed the four bed screws.