I have re-read your post three times trying to see if I can offer any advice, and regretfully, I still can't figure out what your situation is.
1) You say, "We received a default judgment in 1998 for a medical bill.". Yet, the balance of the message says that you are paying (or have paid) this judgment. If correct, you didn't 'receive' this judgment (being the plaintiff), but LOST the case and a default judgment was rendered AGAINST you. Which is correct?
2) You further say, "We cooperated with the attorney by making agreed monthly payments. For no reason we were called
in for interrogatories." In reading this, I assume you negotiated a payment schedule with 'some' attorney (assumed the judgment creditor), but that he apparently called you in for a 'debtors exam'. Normally, if a payment plan is in place and being followed, the creditor wouldn't just arbitrarily incur the cost of a debtor exam. Why did they call for one??
3) You say, "We reviewed our current medical bills we owe without reviewing any of the requested docs he asked for."
How did you manage to avoid providing documents for a debtors exam?? They are normally requested by subpeona and it would be REQUIRED that you bring them.
4) You say, "He said he would settle for the principle amount and requested we submit a letter outlining our circumstances, we complied."
I thought you were already making payments. Why would he make an offer for the principle amount when you were already making payments??? Were you only making payments on the interest and a balloon payment on the principle??
5) Then, after all the talk about lawyers and exams and making payments, you immediately jump to, "6 months later after making our final payment we received a letter from the Doctor stating our account was paid in full with a $0 balance.". Making your final payment on what?? On the principal, the interest or both?? Why did you receive a letter from the doctor when all of your earlier conversations were with a lawyer (for the doctor?)??
6) You say, "8 months after the receiving the Doctors' letter the attorney sent a demand for payment letter on the interest which has far exceeded the principle amount and attorney fees over $500.00."
If you will read the judgment, you will see that it very probably includes the original debt, PLUS attorney fees, PLUS interest, PLUS costs. If you have only paid the original debt, you are still liable for the remaining costs of judgment.
7) Finally, you ask two questions, "Can they reverse their agreement with us, even though their actions implied it was in place and is it legal to charge two different interest rates (mine was 9% and my husbands 8% combined with 18% up until the judgment was placed)?"
It depends on the 'agreement' that you claim. If the agreement says that you will pay $X as payment in full, and they now demand more, generally, you will not be required to pay additional fees. It all depends on this 'agreement' you allege. Look at it and see what it says. As to the interest question, your post is again confusing. How can ONE debt have two separate interest rates? Was the judgment a consolidation of TWO separate medical bills, one at 9% and one at 8%? If so, then each bill should have been brought forward to the date of judgment, with its own pre-judgment interest, then a new post-judgment rate could be set by the judge as allowed by state law.