Hi,
There are no fees when transferring or consolidating your accounts.
However two points to note. During the transfer period your investments will not be invested in the market, i.e. they've been turned into cash for transferring to your new provider, so you won't participate in any ups or downs in the stock market. Also note you may be transferring from a cheaper MPF fund to a more expensive one. Or vice versa. Though over a six month period it shouldn't make that big a difference unless you have a lot of money in your MPF's.
Perhaps more importantly if you have a lot of different MPF accounts all over the place you may want to consolidate them so that when you do want to withdraw it you only have to contact one MPF provider and not all the separate ones individually.