Provided the sound card has a place to plug into, it shouldn't be a problem. Mine allowed me to plug in my headphones at the front AND at the back at the same time, but then I got an Audigy but it didn't have an analog output
But anyway, you should have nice little wires attached to the onboard soundcard - just plug them into the new one, and it should be fine.
I've seen some dumb as front panels, I set up one where the connection to the front actually had to plug into the headphone jack of the soundcard itself, so it had a stupid wire coming out of the back
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