I wonder if it'll be a gradual change - possibly dual branded versions of the ITV idents in the spirit of the 1989 and 1999 network packages, then quietly retiring the UTV name over time.
I doubt it. Both those packages were a compromise between still independent companies keeping their own brands and also creating a cohesive network identity at the same time. Neither were without problems regarding implementation either. UTV will be wholly owned by ITVplc and so no such compromise will be necessary. I reckon it'll be more of a Grampian 'here today, gone tomorrow' sort of thing.
It probably won't happen on the day of the merger, although generic news titles for both regions and the network news were introduced on the same day Granada & Carlton merged. How it all pans out, in what order and what time scale will be interesting. It'll be like going back to the late 90s/early 00s again. Will they remove the continuity at the same time as the branding? Will transmission be moved too or will it get a stay of execution, at least for a short while, a la Central? Time will tell.