Seymour has won all of his 43 Scotland caps on the wing but has occasionally covered full-back during his career and is recalled in the No 15 jersey, with Ruaridh Jackson dropping out of the squad following last Sunday's Pool 3 opening loss 13-3 at home to Saracens.
Seymour has had some time off following a bereavement and Rennie believes his experience will give something approaching the X-factor of the currently injured Stuart Hogg.
Speaking in The Scotsman, Rennie said: "He is lighting quick, similar to Hogg. He is really strong aerially and we know Cardiff are going to kick a lot of ball, they kick second most in the PRO14.
"He has a really strong kicking game and is a good communicator. He has not played a lot at full-back but what it means is that he will get a lot more ball in his hands and will be able to ask some questions from a counter-attack point of view."
Glasgow's other change sees Australia-born centre Sam Johnson, who was called up for the Scotland autumn Test series this week, come in for Alex Dunbar, who sustained a head injury during that bruising battle with Sarries.
"Alex picked up a knock and would still be going through protocols," explained the coach. "We decided to make a call early in the week and bring Sam back in. At a push, Sam probably could have played last week but we wanted to make sure he got through a lot of rugby training, got the ball in his hand and got through a lot of contact work.
"Alex probably would not have been available and Tommy at full-back, he probably offers something a little different that will be good for us this weekend."
Rennie will unleash the same pack of forwards who fronted up so impressively against Saracens' monster pack as Glasgow look to ignite their European campaign on the back of a frustrating but far from dispiriting start,
"We were really happy with our set-piece," said the coach. "The defensive maul was excellent, we scrummed well, the lineout functioned pretty well. Then, around the paddock, we made an enormous number of tackles from our front eight. Bodies are pretty battered but we are pretty happy with them, that is why we are putting them straight back out there. It is good from a continuity point of view."
Tomorrow's encounter against familiar Guinness PRO14 opposition in Wales looks, on paper, like a totally different proposition to facing Saracens but Rennie is wary of viewing it that way.
"It is funny when you look at games. We had opportunities to play last weekend," he said. "We shortened the defence up and had a chance to play with a bit of width.
"When we shared that with the boys, we had a few opportunities that might have put them under a bit more pressure.
"There is no doubt that Cardiff do kick the ball a lot but they also have players who can play and sting you. We always want to play an up-tempo game but also want to make sure we are going forward and earning the right to use the ball."
Rennie conceded that losing at home in the first pool game makes tomorrow as close to "must win" as you get.
"It is pretty tough, isn't it? If we lose this weekend," he said. "It is not impossible after the Scarlets did it (losing first two games and reaching the semis) last year but what it would mean is that we would definitely have to beat Saracens away from home and pick up some bonus points as well as looking to win every game.
"It will be pretty tough and we are well aware of that.
"We are not really talking about whether we win or lose, it is the quality of the performance and our ability to put on a good show.
Glasgow won 20-19 on their last visit to the Arms Park on PRO14 duty last season but Rennie is expecting the Heineken Champions Cup context will add something extra,
"They have a few different (coaching and playing) personnel," he said. "It is always an arm wrestle down there. We only just pipped them in Cardiff and I was down there the year before (watching) when Cardiff beat the Warriors. They are a good side, they had a pretty good season last year after a slow start and they are pretty similar this year, three losses and then in pretty good form.
"Our focus is on us. We have got to play with the same intensity as we did last week but be more clinical, more patient. That has been the focus this week."