Jagex used to allow players to simply update the expiry date without purchasing but they stopped that a few years ago.
Now, you have to buy something to update the card.
If you buy a new subscription, it will auto-cancel the old one.
This should not affect you UNLESS your account currently has one of the obsolete subscription types.
3 month regular is no longer offered.
12 month regular is also no longer offered.
The current choices are:
1 month (with grandfathered price)
6 months (with grandfathered price)
Premier Membership (12 months - new product - new price - not grandfathered)
If you have one of the obsolete membership types and
really
don't want to lose it you will probably have to buy something different (Bonds/keys/RuneCoins etc.) to get your new card into the system without compromising your membership choice (particularly if you have the old 12 month regular with grandfathered price
)