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The wearing of white underwear during the clean days serves two purposes: it demonstrates that the woman is aware of her seven-day count and it reveals any bleeding that might invalidate the clean days. From the hefsek taharah until sunset, the first reason would seem moot, since the clean days have not technically begun. As long as a moch dachuk is inserted, it would seem that the requirements of the second reason are satisfied. Still, the prevailing practice is to change right after performing the hefsek taharah. Definitely, you should make every effort to change by sunset.
On the go, this can be accomplished as follows: wear white underwear on the day you expect to do your ;hefsek, with a pad to catch any stains. After the successful hefsek taharah, remove the pad to reveal the white underwear. If, for logistical reasons, changing earlier was impossible, the clean days are not invalidated. But the best practice is to change or to prepare as above.