Just pulled down the latest Communitas 3.17 version for Civ 5 BNW and Played the French. On version 3.16 once I researched the Gatling guns I could upgrade Musketeers & Arquebusiers to Gatling guns since they are classified as Archery units. I also posted this update in a second link because I am not sure which link you prefer 3.17 bug updates with -
http://forums.civfanatics.com/showthread.php?p=13345823#post13345823
In CEG version 3.17 Muskateers & Arquebusiers are no longer up-gradable, but Crossbowman are upgradeable to Gatling guns. So this looks like a feature regression. And by not upgradeable I mean they will not upgrade to musketman or gatling guns. They won't upgrade to anything in the Industrial Era.
Repro Steps:
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1.) Install Communitas 3.17 into the Civ 5 mods folder.
2.) Install IGE latest version into the Civ 5 mods folder.
3.) Start a Mod game in the Industrial Era as the French.
4.) Use IGE to give yourself the tech updates necessary for Gatling Gun (Steampower), and 2000 gold to purchase unit upgrades.
5.) Use IGE to give yourself a Crossbowman, a Musketeer, and an Arqebusier.
6.) Attempt to upgrade the Crossbowman, and notice that
it will upgrade to a Gatling gun.
7.) Attempt to upgrade the Musketeer, and notice that
it will not upgrade to anything (no upgrade button displayed).
8.) Attempt to upgrade the Arquebusier, and notice that
it will not upgrade to anything (no upgrade button displayed).