Resolution Upgrades
Generally speaking, subjects with highly reflective surfaces require higher resolution versions of environment maps or the reflections will suffer from pixelation. If the subject is matte, or if the surface blurs the reflection then a lower resolution file will suffice. If you find that you need more resolution than your Sampler files provide, you can purchase a 'resolution upgrade' to a higher resolution of the same file and get full credit for your earlier purchase. For each of the six Sampler files, the credit is one sixth of the bundle price $12.
Note that you can't upgrade to a different product, only to one that you have in the Sampler bundle you purchased.