The medal roll, which is the document you are referring to, is showing the change of numbers in early 1917 for men of the Royal Field Artillery Territorial Force. RFA T is simply another way of writing Royal Field Artillery Territorial Force, there was no morphing of one to the other. (The inclusion of "Horse" there is simply because the same record ofice dealt with both Royal Horse and Royal Field Artillery). The roll is not showing the units to which he belonged within the RFA, that is where the research comes in.
The new numbers can be traced to the unit they were issued to, and in his case we can say that at that time he was in 303 Field Brigade.
We can't say he was in that brigade when he went to France in 1915, not least because that brigade didn't go to France until June 1916 so the next thing is to find out who he was with then.
303 was a London brigade (hence my query) and was, in 1916, in France with the 60th (2/2 London) Division. That division was the reserve division for the 47th (1st/2nd London) Division who went to France in March 1915 (same as our man).
They took with them London RFA Brigades, 235 from Kennington and Paddington, 236 from Brixton, 237 from Fulham and Shepherds Bush and a brigade first called the 8th London (Howitzer) Brigade which, in May 1916, became 238 Brigade and which then in March 1917 was broken up.
As his service record is missing as you know, it is not possible to prove/disprove which brigade he went to France or when he moved from wherever to 303 Brigade where he was renumbered.
However, the war diary for the artillery of 47 Div records 8 Brigade detraining and going into their billets at Ferfay on 18 March 1915 which indicates that they arrived in France a day or so before that (he arrived 16 March). The unit breaking up in 1917 suggests he had to find a new unit which would account for him being in 303 at the renumbering. More research may give more solid evidence.
maxD