The speed restriction on the STAR remains until ATC cancels it. If they want you to maintain 280kts on decent they would say "Descend not above 280kts, cancel speed restriction on the STAR"
If not cleared via a STAR ATC will say "cancel speed restrictions above and below 10000' "
If given a speed restriction of 230kts, it then become limiting. Therefore you can't increase to 250kts.
hope that answers your Question.