in 2004 trials can you drop down a distance and move up a distance?

Hmmmm....Not exactly.

Was this really a rule in the past? People could compete in the trials in a distance other than what they qualified for?

I remember hearing about the rule for the 2000 trials but I don't think it applied to the marathon. Although if I remember correctly Wejo ran the marathon, the 10k and the 5k but I think he only had a 10k and marathon qualifying time. Don't quote me on the last part though.

it was a rule, which a few people used (fairly, in my opinion) to their advantage. if anyone could shed some light on this, it would be great. thanks.

I checked the USATF site and couldn't find anything about it.

I know it's a rule for the junior competition, to qualify for the 5k, for example, you can use a certain 1500 or mile or 3k time....

As far as I know, the rule was that if you qualifed for the Trials in two events and completed both of them, you could also compete in a third event of shorter distance. I'm not sure if this only applied to the distance events.

For example, someone who competed in both the marathon and 10K (like Wejo) but didn't make the team, would have the opportunity to compete in the 5K.

Teddy Mitchell might be able to clear this up. I remember seeing him leading one prelim of the 5K, and I doubt he had run under 13:50 that year to get the qualifier.

Track and Roads. Two different disciplines. Marathon times are not used in consideration for Olympic Track Trials entry. Can you imagine most (100+ bodies for the men) of the Marathon Trails qualifiers wanting to enter the 10,000 on the track? That would require quite a few prelim heats or some rather Mt. Sacesque size starting fields. Not to mention a rather large differential between the competitors abilities (say 27:20 vs. 30:00+) generating a lap fest. The men's 10,000 field is limited to 24 entries unless there are more than 24 under 28:15.00.