I, personally, have strong feelings about this, and I know others, including some local brewers do too. Let me try and explain.
The simple place to start is a brewer brewing a new beer. They make a full brew length, appropriate to the brewerys size, give it name and distribute it to pubs. The beer comes on the bar and the drinker drinks it. A new beer obviously. The confusion starts when the brewer has a couple of barrels of beer left and someone approaches the brewer for a festival special. What happens then?
Does the brewer give them the original beer with the original name, the original beer with a new name (cheating the tickers surely), or add something to the cask to make the original beer different and give it a new name ? This , in my humble opinion, would make this a new beer. Often, and usually the easiest thing to do is add some hops to the barrel, and therefore 'dry hop' the beer, which alters the flavour of the original beer, (and therefore makes a new beer).
A couple of years ago, as an experiment, and with the help of a local brewer, Pete Byrne and myself brewed a beer. A full brew length that was released into pubs. We kept two barrels back for a local festival and dry hopped each one with different hops. These appeared on the festival bar, along with the original, with nothing to suggest their provenance. We listened to feedback from the festival goers. Only a hand full sussed out what we had done. Most assumed they were different beers, they certainly tasted different to each other. Therefore, I am quite happy to call a dry hopped barrel of beer 'a new beer'. This covers most of the festival specials around.
What annoys me is a brewer just rebadging,(using an old beer and just giving it a new name for the occasion) a beer to satisfy the needs of the festival, there is often nothing to suggest this is the same beer that I have been drinking under another name, and seems to me to be a way of misleading the 'ticker'.
If you are familiar with the beer drinkers bible, the GOBBS guide (c), a quick scan will show some breweries that use several names for the same beer, and unless you are prepared to do the necessary research, then it is easy to assume all are different. I cannot understand why some brewer should want to try to mislead the drinker by renaming a regular beer as something different. I know of enthusiasts that will not drink certain breweries beers because they are unsure of the beer's provenance.
I know this may sound a very pedantic post, and it affects only those of us sad enough, sorry, enthusiastic enough, to want to list every beer we have, and want every beer that has a different name to be different. I have no issue with 95% of the beer I come across, or 95% of the brewers. But with Xmas coming, and the Royal Wedding on the horizon can I be sure that every beer I drink is actually a new beer, or is it an old favourite renamed. I certainly hope its not the latter.