On PartnerSource or CustomerSource, I would recommend looking at the 'Upgrading to Microsoft Dynamics GP 2018' hot topic first, to verify the upgrade path you plan on taking from GP 2015 to GP 2018, to make sure it is actually valid, as in, depending on what version/build of GP 2015 you're currently on, dictates what version/build of GP 2018 you can directly upgrade to.
I'd also look at the upgrade guide for Dynamics GP, as it has pre- and post- steps in there to prepare for an upgrade of Dynamics GP, as well as 'known issues'.
Make sure you've gone over the System Requirements for Dynamics GP 2018 to make sure you're migrating to a supported Windows and SQL version, along with other minimum specifications required for GP 2018.
If you're migrating the GP databases from one server to another, I'd recommend using KB 878449 which you can find online. This article has the steps and scripts needed to setup the new server/SQL Server to allow Dynamics GP to run on it.
Last but not least, we ALWAYS recommend a test upgrade, that way you know exactly what you're running into and can have the resolutions in place before the 'live' upgrade, thus minimizing down time for your customer.
Let us know if you have any questions.
Thanks
