Upgrading an app from 5.4.3p3 to 2017.3.1f1 missing graphics

Hi All,
I'm updating an old game, and wanting the benefits of all the latest optimisations with the past few years of Unity upgrades, but on getting the game to work we now have missing graphics, has anyone else experienced this, our game now only shows textures that have an alpha channel, no opaque textures render at all.
Cheers.