50 pence probably won’t make a lot of difference to the take-up, though bear in mind that Patreon already charges members £4.20 for the £3.50 subscription. I don’t recall Patreon ever stating that anywhere, though it’s now a while since I started it so I may have just forgotten.
However, it’s usually best not to change three things at the same time (ie, the new forum, age restrictions and the price) otherwise it makes it harder to judge whether lost members are down to migration, unwillingness to submit to age restrictions or the price.