Will cause no problems - dont remove the old one though I would suggest.
Yes. Do not move it at all.
Let PayPal do that for you whenever they notice, I made that mistake and my account was limited because they suspected suspicious activity with my Credit Card.
You will not usually be able to remove the other card. paypal tend to send a message that a card can not be removed pending transfer of funds. (This may not be actually the case but it's a message they send.) Appears to be paypal protecting themselves.
__________________ REAP WHAT YOU SOW. LIFE IS SO NOT FAIR.