I would do that as well, but it's not because of some deficiency in Jackson so much as the significant upside with Martin.
IMO, Jackson is being seriously slept on this year, and he's going to have a really good year. People worry about Jackson's switch to a run first team, but forget that in San Diego, with all of the other pass catchers there, he really didn't have that many receptions and he was still productive. Jackson always had Gates and a rotating cast of quality receivers who would get the ball plenty in that offense. While Tampa's offense will pass less, that strong running game is going to give Jackson a lot of opportunities on play action, and he's option 1(a), 1(b) and 1(c) in the passing game. Jackson is a guy you can get in the 7-8th round who will put up top 20 WR numbers, IMO.
All that being said, Martin is likely to be a top 12 RB. There are a ton of good WRs out there who will get you solid numbers. There aren't that many RBs who look like they're going to average 20 touches a game. Martin looks like he will. While Jackson has great value in the 8th round, Martin has good value in the 4th. If you can swap for the two, that's a win.