A couple of things:
You need to get those to 2007 SP3 plus the latest UC if possible first if you want to eventually migrate to Exchange 2013 - then to Exchange 2016/2019.
Personally I would build new servers. Since 2007 is out of support, there is no public , official way to get those latest service packs and UCs, So unless you have those available to you, you are stuck.
As far as the current FFL/DFL, I read it the same way, it should hopefully work for you now and if you upgrade the 2007 servers, but you are in completely unsupported territory, so no one at Microsoft can help you if something goes sideways.
You may want to also consider third party software and simply export the existing mailboxes out and import them back into upgraded servers
Or use the built-in export powershell in 2007:
https://techcommunity.microsoft.com/t5/exchange-team-blog/how-to-export-and-import-mailboxes-to-pst-files-in-exchange-2007/ba-p/594262
- or use exmerge which probably worked on 2007 : https://www.microsoft.com/en-us/download/details.aspx?id=2743
Alternatively, just give them new mailboxes on a supported system and not attempt to upgrade.