You can edit your card info in your PlayNC account, so I guess you input the details for the new credit card, buy your slots and then edit info again and put the old one back for your subscription.
You know, I just missed that whole Billing page. 
Thanks Nightmarer.
"Idealism is such a wonderful thing. All you really need is someone rational to put it to proper use." - Kerr Avon
Myopic Aardvark on Twitter
No worries mate 
On the other hand, if you want to change your payment plan from 1 month to 3 months, then you have to actually cancel and reactivate your subscription and you'll be charged 1 euro as I just discovered 5 minutes ago
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You know, I just missed that whole Billing page.
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Were you...myopic? 
@Golden Girl
City of Heroes comics and artwork
Right. I'm wanting to add some slots, using a different card from the one that I pay my subscription from.
Is the only way to do this to cancel my subscription, change the card details, log in, add the slots, then cancel the subscription and go back to the original card details again?
"Idealism is such a wonderful thing. All you really need is someone rational to put it to proper use." - Kerr Avon
Myopic Aardvark on Twitter