11th London and KRRC

[alibar]

I wasn't entirely sure where to put this post. My grandfather Ernest John Greave born 1898 Newington was in both these regiments. Number 453153 11th London and 48895 KRRC. When would he have enlisted given these reg numbers? Why the change in unit? I know he was in Gallipoli and then sailed via Malta to Palestine. We have a range of photos and his personal diary started in Dec 1917 on SS Abbassieh to Malta. It's very difficult to read as it's in pencil on very thin paper. Any help on my questions would be gratefully received.