My understanding is that investors can use one rental property to purchase or improve another. And the interest is tax deductible for acquisition debt (real estate), but not for personal debt (cars, credit cards, college). The laws are different for rentals and primary residences.

Yes--no longer deductible. Pretty sure this is one of the specific examples the IRS gave in their clarification about what is and isn't deductible. But if you used it to buy your primary residence and just converted it to a fixed rate this year, it's worth looking at whether it's more beneficial to get a mortgage on your primary home and then pay off the HELOC. The rates likely aren't that different, and then the interest is fully deductible.

OP here with another question: Do expenditures prior to the new tax law (December 2017) qualify for any exemption or grandfather clause? This is old debt from 2012 that we’ve been able to deduct until now, not new personal debt. Is the party over?

That's my understanding as well. The interest deduction for an investment has to be related to improvements toward the investment property to be deductible. Otherwise, in your situation, it would be a dodge of obtaining a deduction for a personal expenditure.

In 2012 I used a HELOC on an investment property to purchase my primary residence. That debt of $90K was converted from a home equity line to a fixed loan in 2018. According to the new tax law, am I able to deduct the interest? If I were using it to improve the same property (the rental home), I know I can. But I used this toward purchasing another property.