Internal and external application form and process is all pretty much the same tbh - they all have to reapply, and then still have to do the form, audition, interveiw etc. I think the only realy difference is obviously they know the kids, and they have their auditions etc earlier and found out earlier (internal for yr 12 music had interveiws and auditions in December and found out the end of Jan).
There;s not that many of them that do change strand, but they did hold some 'taster' days in October I think it was for anyone thinking of it which was good. I think most of the kids that did look at changing strands though were keeping in either the creative arts subjects or performance arts subjects - so say theatre to mt, or vad to da, or fmp to production arts - so somthing a bit more logical. My daughter has gone from VAD to music which are obviously very different - don't think they have that many change between them!
They were supportive of the strand change but did also make sure that she knew it was a big big risk, and suggested she apply elsewhere as backups, so she also applied to Access Creative and Rythm Studios and had offers from them. Luckily it went her way and she's been offered music at Brit but I definitely have more grey hair now!
What is your daughter looking to change from/to? I'm sure mine would be happy to chat to her about the strand change stuff if similar - I know she had a good chat with a year 12 and a year 13 that moved to music before she applied and found it helpful.