Cavalry guard  => Scots Greys  => Royal Scots Greys  (as good as French cavalry but with a high pricetag)
Longbowmen  => Rangers  => Green Jackets  (skirmishers, upgrades to line infantry. The Longbowman has an enhanced rate of fire but is costly, while Highlanders are so tough they can make for line infantry. The ultimate upgrade, the Green Jackets are on par with Native Americans, with the ability to move stealthed and with no min range, but are costly) => Black Brunswick Fusiliers? 
Redcoats  => Regimental Redcoats  (line infantry, dressed in the traditional red garb, with good damage and armour, and like the longbow, with a withering rate of fire that causes lesser mortals to melt...but a compensatory pricetag)
Foot Guards  => Grenadier Guards , Royal Marines [?] (a vastly superior grenadier with a higher pricetag. Grenadiers are as good as the Royal Musketeers, although by  the line infantry line will lose minimum range)
Black Watch [3 & 4] => Gordon's Highlanders  Excellent, all-rounder infantry - can't entrench like Marine units, but grants a balance between the power of grenadiers/marines, the range of snipers/marksmen and the cost of line infantry.
Yacht  cheap and strong light vessel of the Colonial Era.
Rocket ship  shared with India, fires rockets that set ships on fire. Replaces anti-ship units.
Cathedral - performs taxation and religion research, and is the main stopping point for your auxiliary units as well as chaplains, which have mediocre attack, but super defensive and defence-boosting skills