I think it could all be possible, as the baptism for Elizabeth Terry as first child on 21 Apr 1728 fits in nicely before the baptism of John Terrell on 11 Feb 1729/30.
There might be a sea change about that time.
From a very basic search on FS, it's Terry all the way in Cowfold from 1700-1728, and then for more than three decades they seem to have no Terrys indexed there at all.
In that period it's all Terrels and Tirrells and even Tirles (do they fit in?). Some of these would surely have been Terrys previously? Burial of a Richard Terrel in 1730, a Susanna Tirrell marriage in 1743, a Susan Terryll burial in 1747, etc.
I have not looked at images!
Could there be a link between a new vicar and a possible change of spelling or interpretation of the Terry name ?
After a long gap, there are three baptisms and three burials for Terry people at Cowfold from 1762 to 1780:
Baptisms:
Ann* 1762 d/o Richard & Mary
John 1765 s/o John & Christian
Sarah 1771 d/o John & Christian.
Burials:
Hanna/Hannah 1774
John 1777
Mary 1780.
I was thinking along the same lines as you jonw65, re: the new vicar and his spellings, thanks for looking at that & suggesting it.
*The baptism entry on Family Search for Ann has Terry, from the PR. However, there is a note on the Sussex Family History Group Data Archive (for members) which states "BT has Tirrell as surname." That is very good news, I think! If that applies to the much earlier Terrys, then we might be getting somewhere.