I have a NUC5i5MYHE running R.O.C.K.
Performance in terms of playback, scanning and remote controlling seem fine but the unit itself (fan) is getting louder. The NUC is in an open rack where the air temp stays between 73 - 78 F.
Since there’s no risk of losing my library or metadata (music files are on a NAS and Roon backs up the database consistently without errors to the NAS).
Should I be proactive and replace now or just wait until playback or control performance head south?
I’d take it to bits and give it a clean before you do anything else. They can get clogged up which disrupts the airflow so the fan has to work harder.
Thanks.
I should have mentioned that I did do a thorough cleaning.
In that case I’d wait until it annoys you too much 
If you are comfortable with PC hardware, don’t forget that after enough time (years) the thermal paste / thermal pad conducting the heat from the CPU to the heat sink may break down. If you are low on money and willing to do that much disassembly, that’s something to try if cleaning isn’t sufficient.
It’s also possible the fan is worn out and just needs replacement.
Replace the fan.
Other option is maybe find a Fanless chassis for your model
Yes, that’s even better.
I had an issue with the fan of my NUC6i.
I just ordered a new fan on AliExpress for 4€. You need to see the reference wrote on the fan and then you could find easily a replacement part.
The problem with fanless chassis is they are difficult to find for older NUC.
And to finish, Don’t wait to replace the fan because in my case the nvme disk didn’t like that the fan didn’t work and it died too.
My Akasa Plato MC has a thermal pad that sinks my M.2 SATA to the case.
Heat is the electronics killer