Melanie is correct about not losing the important stuff, like bio, dates, and attached persons, but there will be some of your WikiTree statistics lost. I'm pretty sure that your contributions will reset to zero and I would guess your G2G profile might also lose its history of up/down votes, questions asked/answered, etc. I'm not sure what else might be reset, but as Melanie aptly said,
someone will surely come along and say so.