The strings you want to put on are lighter (40-100) than the ones you've got on there now (45-105), not heavier.
The truss rod might need a tweak, but it all depends on the individual bass - I've had basses where I've never needed to adjust the neck & ones that needed adjusting all the time. It's easy to do, but if you've never tried it before you'll probably be a bit nervous about doing it.
There are probably other forum members in your area, why not see if somebody can go through what you need to do with you so you can learn to do it yourself? It's much cheaper than paying for a set up every time you get new strings.