The Sage 50 Canadian software will not convert 'down' to a cheaper 'Edition', only 'up' to a more expensive one, or 'across' to the same 'Edition'.  There's no support from Sage Software for 'downsizing' from 'Premium' to 'Pro', or from 'Quantum' to 'Premium'.

The quickest and easiest thing (but not least expensive) is to exchange or upgrade to the Premium software.  Unless there is some feature in the 2018 software that you absolutely need, the least expensive thing to do would be to continue to use the 2015 software if the license allows it.

Otherwise, it's possible to export the 'lists' from the 2015 software, creating a new 'Company File'  in the 2018 'pro', and import the lists, then complete the setup as you would for any newly created company file. 

There are services that claim to be able to convert your data from a 'higher' to a 'lower' Edition, as long as you didn't use / aren't using any of the 'premium' features, it is certainly possible. 

I hope that helps, please post back.