What is Changing?
Under previous tax law, if you converted a traditional IRA to a Roth IRA and then those investments fell substantially in value, you could reverse the conversion and turn the Roth back into a traditional IRA, this is also called "recharacterization", then you can reconvert the traditional IRA to a Roth IRA with the value of the investments - and the resulting tax bite - lower than when you did the original conversion, therefore paying less tax.
The new tax law eliminates this option to recharacterize, which makes converting a traditional IRA to a Roth IRA a riskier strategy because you cannot reverse it.
What to Do?
Consider spreading the conversion to a Roth IRA into equal portions over a few years. In this way, you will pay less tax on a portion of the converted assets if the value of those assets drops before you convert them.