Is there any checklist to configure before upgrade for better deciding?
Is there any checklist for it after upgrade?
I have used the Upgrade status module on the past to better understand how much work will be involved in upgrading a site from a mayor version to the next.
Also, depending on how many modifications, or modules you have, it will also make the process easier.
One more tip: Check the Drupal documentation, and remember Youtube is your friend.
Just follow the 20 steps for successful upgradation from Drupal 6 to Drupal 7. These steps are listed in the Community docuentation page Upgrade process.
Identify the hacked modules and/or themes
Often times when you're considering upgrading you also want to have an answer to the question like "Are there any tweaks (hacks) in any of our contributed modules that we need to be aware of?".
Especially if somebody else first created the D6 site, and you're in charge of the D7 upgrade.
Guess what, there's a module for it, it's called Hacked!. Here is a quote from its project page:
Evaluate module alternatives
When upgrading you might have to replace some module with another module, for various reasons, such as:
Or you may want to take advantage of adding new features, etc in your upgraded site.
So there are quite some situations where you will have to decide between multiple contributed modules, such as:
Whenever I have to decide between multiple modules, I often use the (23) criteria that I ever documented in "maintenance scorecards" (within the Community documentation about "Comparison of Charting modules"). Those scorecards are actually "applied" to charting modules, but the same criteria can be used, I think, to mostly any type of module comparisons.
D8 module status
The D8 Module Status site provides an interesting summary of various contributed modules, about their D8 readiness. Here is a quote about it: