The strings I build have a bunch of tight twists near the string loops that go 5-6 twists more than needed to hold the tag ends of the bundles. These tight twists stay in place when I untwist the string. If your string also has these tight twists, then you can untwist the main part of the string quite a bit without compromising the string. If your string does not have these tight twists toward the end, then I would not untwist it past the point that it has 1 twist per 2”.
Note that the serving will still have the original twists underneath, so there will probably be an imbalance between the twists under the serving and the twists outside of the serving. As these equalize over time, it could affect the serving in some way, possibly necessitating a replacement serving, I really don't know.