Because if you have a limited upgrade, and want to replace it with a different limited upgrade, you must first discard your limited upgrade to be able to attach his other limited upgrade. In other words, to attach a limited upgrade, he must first remove his current limited upgrade. If isn't attaching a limited upgrade and instead is attaching is 1 non-limited but still shenlong restricted upgrade then his current limited upgrade card, assuming he has one, isn't blocking it's attachment.
Also, "may" could easily be removed and the language would still be clear. "To" would need to be replaced entirely with some other word or phrase or what's remaining is a grammatically incorrect sentence that has no clear meaning.
To explain it another way without adding in extra words. If you have a limited upgrade and you wish to attach a different limited upgrade, you may discard your current limited upgrade to attach your desired limited upgrade. If you don't discard your current limited upgrade you are blocked from attaching your desired limited upgrade because you can only have 1 limited upgrade attached at a time. Your current limited upgrade is blocking your desired limited upgrade, so first it must be removed. If you have a limited upgrade and wish to attach a non limited upgrade, you may discard your limited upgrade to attach the desired non limited one. If you choose not to nothing is blocking explicitly blocking the non-limited upgrade(assuming you don't have 3 upgrades attached at the time).