For the transition from one service pack to another, no worries.
But when changing the version, it is strongly advised not to update.
It is rather advisable to redo a clean installation next to it.
Let's imagine that we are on Solidworks 2014 Sp4 and that we want to switch to Solidworks 2015 Sp0, while keeping the 2014 version just in case (which is recommended during new versions that are not yet too stable)
You have to install the 2015 next to the 2014 and rename the installation folders of the 2 versions.
Solidworks data 2014 and Solidworks data 2014. The same goes for the toolbox. This avoids version conflicts and incompatibility issues.
Afterwards, when the old version is no longer used, simply delete it.
In any case, this is what Dassault recommends doing.