I want to open a 529 acct with me as beneficiary, move unused funds from my son's 529 that he doesn't need, to pay my student loan. Will that create tax liability?

You can rollover your son's 529 plan to a new 520 plan with you as beneficiary. This rollover isn't taxable as it is a rollover for the benefit of a beneficiary's family member.

See this IRS document on page 53.

You can then use your 529 plan to pay your student loans up to a lifetime limit of $10,000 without having to pay taxes.

Q. I want to open a 529 acct with me as beneficiary, move unused funds from my son's 529 that he doesn't need, to pay my student loan. Will that create tax liability?

A. No.

Check with your plan administrator.  It may be a simple matter of just changing beneficiaries in the existing plan. 

There are no tax consequences or penalties when a 529 plan beneficiary is changed to a member of the beneficiary’s family. Qualified family members include the beneficiary’s:

1. Spouse
2. Son, daughter, stepchild, foster child, adopted child or a descendent 
3. Son-in-law, daughter-in-law
4. Siblings or step-siblings
5. Brother-in-law, sister-in-law
6. Father-in-law, mother-in-law
7. Father or mother or ancestor of either, stepmother, stepfather
8. Aunt, uncle or their spouse
9. Niece, nephew or their spouse
10. First cousin or their spouse

https://www.savingforcollege.com/article/how-to-change-the-beneficiary-on-your-529-plan