I honestly believe that questions like this one: Using GPO in Active Directory domain to force workstations Windows Firewall to disabled - how? existed because Windows Admins in general were taught long ago that:
"the easiest thing to do when dealing with a domain computer is to just have a GPO on the domain to disable the Windows Firewall...it will cause you much less heartache in the end." - random IT instructors/mentors from years gone by
I can also say that at MOST companies I've done side work for this has been the case, where a GPO at a minimum disabled the Windows Firewall for the domain profile and at WORST disabled it also for the public profile.
Even further, some will disable it for the servers themselves: Disable firewall for all network profiles on Windows Server 2008 R2 through GPO
A Microsoft Technet Article on the WINDOWS FIREWALL recommends you DO NOT disable the Windows Firewall:
Because Windows Firewall with Advanced Security plays an important part in helping to protect your computer from security threats, we recommend that you do not disable it unless you install another firewall from a reputable vendor that provides an equivalent level of protection.
This ServerFault question asks the real question: Is it alright to turn off firewall in a LAN using Group Policy? -- and the experts here are even mixed in their view.
And understand I'm not referring to disabling/enabling the SERVICE: How can I back up my recommendation to NOT disable the Windows Firewall service? -- so as to be clear that this is about whether or not the firewall service enables the firewall or disables it.
THE QUESTION AT HAND
So I get back to the Title of this question...what can be done to properly re-enable the Windows firewall on a domain? Specifically for client workstations and their domain profile.
Before simply switching the GPO from Disabled to Enabled, what planning steps should be taken to ensure that flipping the switch doesn't cause critical client/server applications, allowable traffic, etc. to suddenly fail? Most places won't tolerate the "change it and see who calls the Helpdesk" mindset here.
Are there checklists/utilities/procedures available from Microsoft to handle such a situation? Have you been in this situation yourself and how did you deal with it?