I'm assuming that the card is just full and you're concerned about the impact of reformatting it and reusing — not that it's failing to the point where the maximum free space is 1% of what it used to be.

Don't worry about formatting. The card is meant for reuse, and even cheap SD cards can go through hundreds of write/erase cycles, with higher-end cards rated for many times that. Especially if this is only the first time you've ever filled the card, reformatting and reusing is very, very tiny amount of wear.

You note that you're transferring the photos to your PC, you have a backup there — you should also make sure you back up those photos from your PC to an off-site location. (Some cloud storage, say.) I'd be a lot more worried about that than about reusing the card.

If you have taken many months to fill up that one card, you could decide to just keep it as yet another backup, buy another, and fill it up too. But that's a relatively expensive way to get backups and not necessarily one I'd particularly trust for long-term anyway. Better off reusing the card (as it is designed for) and putting the money towards online backup.