Some tech advice please!

are the pads still soft or brittle and dry? if they are dry you should replace them. if they are still soft you can keep using them.

Just make sure it makes good contact when you put it back together and it should be fine. It should puff back out a little and then compress when you screw it back on. If it is dried out and doesn't expand at all to make contact, you will probably have thermal issues because of the bad contact.

You need to replace them because (either of or both) :
– the "stamped marks" are made on purpose when the thermal pipes are screwed in, and when you screw off then back it never comes back in the exact correct position, thus you need new pads to ensure good thermal dissipation (even if only one month old)
– you will feel less worried with new ones 😃

yes,but if the memory chips hynix\samsung etc had problems ( because of the 5 years old solder usurated\cold + low quality solder that is habit to be in notebook&consoles,and so you had green\blue screens or video artifacts etc.) i doubt you can fix it by just re-apply new thermal pads or thermal paste,only way to fix is rebuild the solder or replace the chips chups chaps that sux.

As cheap as thermal pads are, and as long as you already have it disassembled, you should replace them.
(edit: also, clean out that fan!)

Either way, as others have said, it will not do any harm to replace the thermal pads with new ones. Make sure you get the same thickness though. Also you will want to clean the CPU and GPU heatsinks with Isopropyl alcohol before applying thermal paste.