A CB radio may use may different crystals to mix and synthesize frequencies to arrive at the correct channel (1-40). You question needs a lot more clarification and detail before someone can provide you with the correct answer.
What is the goal of your project? Are you trying to repair a CB radio or just change the channels?
CB radios operate in the 27Mhz range. Only very old CB's used crystals to set the frequency of each channel and had 2 crystals for each channel, 1 for RX (receive) and 1 for TX (transmit). So a 23 channel CB would have 46 crystals on the circuit board. 40 channel CB's never used crystals for channel frequencies. If your CB has only 1 crystal then it is for the PLL module and has nothing to do with the frequency that the channels operate on. But to answer your question, if you have an old CB that uses crystals then replacing a 27.XXXX crystal with a 6.XXXX crystal will decrease the frequency, not increase it, but since it is a frequency that is not used in CB's you will not receive anything and your transmissions will not be received by anyone else either.
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A CB radio may use may different crystals to mix and synthesize frequencies to arrive at the correct channel (1-40). You question needs a lot more clarification and detail before someone can provide you with the correct answer.
What is the goal of your project? Are you trying to repair a CB radio or just change the channels?
CB radios operate in the 27Mhz range. Only very old CB's used crystals to set the frequency of each channel and had 2 crystals for each channel, 1 for RX (receive) and 1 for TX (transmit). So a 23 channel CB would have 46 crystals on the circuit board. 40 channel CB's never used crystals for channel frequencies. If your CB has only 1 crystal then it is for the PLL module and has nothing to do with the frequency that the channels operate on. But to answer your question, if you have an old CB that uses crystals then replacing a 27.XXXX crystal with a 6.XXXX crystal will decrease the frequency, not increase it, but since it is a frequency that is not used in CB's you will not receive anything and your transmissions will not be received by anyone else either.