Are you sure we can't interest you in a VBA solution? [;)]

Okay, if your list is sorted as in your example, then try this. For
example, place the original list in a column  starting at A1. In B1 place
the formula
=IF(A1>50,100,A1)
For the rest of column B (starting with B2 and dragging down) use the
formulas
=IF(AND(A2>50,MAX(B$1:B1)<100),100,A2)
=IF(AND(A3>50,MAX(B$1:B2)<100),100,A3)
...and so on...