In CF3 running on Joomla, I am able to do a dynamic validation using the code below
I have only a little experience with CFv5 on WordPress; that said, I would suggest that you switch to a different jQuery validation library to do dynamic validation. The ChronoForms library sets up the validations on form load and I have never found a good way to over-ride them after that*. While it may be possible it probably isn't worth the work involved. I have used jQuery Validation successfully before.
* The exception is that disabled inputs aren't validated but I don't think that helps in your case.
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