Do we need to disable the trim on an 3rd party SSD before updating a Mac? I have a MacBook Pro 13-inch Mid 2012 and I installed an SSD in it. I'd like to upgrade my computer, but I'm not sure if I need to disable the trim before the update and reenable it after. I've heard that it could cause problems, but don't know why. 

I have macOS Sierra 10.12.6 and I've enabled the trim since I switched to this operating system.
TRIM is a subset of the ATA commands specific to SSDs.  

Can you operate without it?  Sure, but it will treat your SSD as if it were a spinning HDD - impacting performance and lifespan.

From Wikipedia:  


  The TRIM command enables an operating system to notify the SSD of
  pages which no longer contain valid data. For a file deletion
  operation, the operating system will mark the files sectors as free
  for new data, then send a TRIM command to the SSD. After trimming, the
  SSD will not preserve any contents of the block when writing new data
  to a page of flash memory, resulting in less write amplification
  (fewer writes), higher write throughput (no need for a
  read-erase-modify sequence), thus increasing drive life.


If your MacBook Pro came with an SSD, it's enabled by default.  If not, you must enable TRIM when you upgrade to an SSD.