Yes, you can. Remember that all P2P items you have will be unusable and un-wieldable. If you still have a members item equipped when you turn F2P, you will still be wearing it, but it will have no stats and no uses (e.g. if you have a whip wielded in a free world, it will have +0 stats, and you will be counted as "Unarmed", and so punch and kick anything you attack.) Once you remove it, it will just be an un-equippable "Members Object". Also, your bankspace will be reverted back to F2P size. So if you have say 200/100 bank spaces used, as soon as you remove one item, you will not be able to put it back in. You can avoid this by either making your bank free size again, or by having two of each item, so the space is never removed.