Clare - why are you aiming to do a fairly flat ultra as your first? In my personal experience they're harder as you have to concentrate a lot more on feeding, drinking and forced walk breaks - if you find one with some nice hills in it you get built in walk breaks up the hills, which is when you can also think about eating and drinking, and you get nicer views to inspire you to keep going... Just a thought...
Karen - if you're now changing to minimals then you've got 6-7 months to be used to them, and that's 6-7 months of running in bad weather, bad terrain etc, so by April you should be completely at ease with them.
Graeme - try it and see.... Seriously, the whole point of ultras is that there is no right, and no wrong answers. As the quote goes "we are all an experiment of one" - what suits you probably won't suit me, and vice versa. Also, and interestingly/frustratingly depending on your point of view - what works this time MAY not work next time.... I've no doubt you'll be doing B2B training runs to get used to the idea of 2 long runs in a row - so try one B2B with the same trainers, and then try another with different pairs, see how you react, see how you feel... To be honest, it will depend on the day. If you get through day one unscathed then you'll probably want to stick to the same pair for day two. If you've got blisters etc then changing may well relieve the pressure?