The biggest problem you'll encounter with just buying a new big ring is that your front derailleur can handle the spread from your granny gear to your (let's say) 48 tooth chainring. If you ditch the granny then you'll be fine.
However, check with Giant first. You may also encounter problems with a bigger ring rubbing your chainstays. A 48 should have clearence, but if you put road cranks on there with a 52, you'll probably get some conflict, especially from the inside ring.
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