If your company has a network license, all users must use the same version. If any user activates a new version, a 60 days grace period will follow. After that period, all users must ‘play along’. If the activations of the new versions were removed before the ending of the grace period, everything stays the same.
A standalone license is perpetual, which means you can use it as long as the OS lets you. If you have a maintenance, you’ll receive a new serial and code for the new version, which you can install besides the old(er) version.
So you can have multple version of SketchUp installed on your system.
You can use the trial period of 30 days to decide wether you ‘play along’ with the new version or not, you don’t have to activate it right away!
If you do decide to activate the new version, the old version get the status of ‘on hold’ which means you can not install that version on another machine anymore, since you decided to trade the new version for the old.
Sometimes you skip a version, eg you have version 2016 and a maintenance contract and received the serial and codes for version 2017 and 2018, but never activated the 2017 version. In this situation, you can not install and activat the skipped version 2017, only the latest!