the SOV is usually loosely tied to the Schedule, how do you handle Unit Price items to a lump sum SOV project?

the SOV is usually loosely tied to the Schedule, how do you handle Unit Price items to a lump sum SOV project?  Just add an item and update it each month?  Or base it on an estimated end of project project total?  Are there any special considerations regarding the schedule associated with these items?  Would these require a change order, unless listed a contract contingency?