Both are fine. In reported speech you can often backshift or not, without significantly changing the meaning. Native speakers don't even notice the difference, as it's often unimportant. Example:
John is going to the park with his son.
becomes in reported speech, either:
John said he is going to the park with his son.
John said he was going to the park with his son.
There is a slight difference in when the action takes place. Using the present tense suggests that the action is either ongoing or will happen in the future, while the past tense is ambiguous.
Again, most of the time the ambiguity is not important. If it is important, you'd have to ask a follow-up question:
Has John already gone to the park with his son, or is he going to go later?
It's the same with "until something". Both the present and past tense are ambiguous enough that it's hard to say whether the action is complete.
John said he is going to practice until he can do it perfectly.
John said he was going to practice until he could do it perfectly.
Again, if it's really important to know whether John is done practicing, you'd have to ask a follow-up question