In general, you should upgrade all 3 (left, right, and center) in order to match the timbre of the front speakers. Otherwise, if dialogue or action moves from left to right, the same person or object will sound different as it moves along the different speakers.
With that said, you also have to make sure that you can upgrade the speakers at all. Some HTIB have a proprietary connection that prevents you from upgrading to better speakers. If you were able to use generic speaker wire, then you're set.
also, many inexpensive HTiB sets have amplifiers which expect to output to 2 or 3 Ohm speakers (thats how they get their outrageous power claims). If you attach 8 Ohm speakers, the apparent "loudness" may decline.
A question, the current center speaker i have says Impedence is 3, but the Pioneer SP-C21 says 6, will anything be damaged by using it with my current htib? I read m_vanmeter's post and i interpret that it would just not be loud as it would be. If this is true, will it be equally as loud as the current speaker?