Can I rejailbreak an iPhone that s already jailbroken?

I've jailbroken my iPhone 4s and my girlfriend's iPhone 4s and I'm enjoying it a lot, but she isn't because it isn't running as smoothly as mine. She can't download a lot of the things that I've downloaded. Other than that, her phone is fine. I'd just like to know if I can rejailbreak her phone without unjailbreaking it? If not, then how exactly do I unjailbreak it? Any help will be greatly appreciated!

You must unjailbreak it then rejailbreak it, unjailbreak it by "restoring" it in itunes

make sure everything is saved before restoring because that wipes the phone clean

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