Triggers fire as the action occurs, thus the terminology. Filters will look at the activity history, so a data value changed FILTER will do what you're wanting to, but note that activities are archived after some period of time. And that period of time varies depending on the type of data, some are archived at 90 days, some 180, etc. So while filters will look at history, they can only look as far back as the history is 'live' and not archived data, I think.