Since they don't get any new skill points, it's almost sure that at the next tier you will be able to find much better mercs. The only reason someone want to keep his merc to the next tier is, the luck of gold. Since at the next tier mercs are more powerful, they cost more. Another reason for not keeping your mercs is the equipment. To equip them with a better one, you have to upgrade them (thus invest some money) and since they are below x5 level of the tier, then they will not be able to equipped with the "good" x8 or x9 level items...
General directions: When you reach the next tier change your mercs, do it having in mind what you are looking for (skills/class), always to be x7 level and up, be patient until you find the best ones, equip them the best your pocket can effort. When you reach x5 level buy new equipment for your hero (until then keep the best items you can from the previous tier)...