You can do so. However, if you use green rest packs to recover conditions and then train the players again, be aware that there is cup match on day 1 & 2 next season in unknown time and you may ensure your players in adequate condition to play this. Besides, in next season, you will participate league, Champion League and Cup. So from day 3, you may need to play 2 games per day (if your team advance to next level in cup after day 2).

For player buy & sell, I think you have time to experiment & learn and need not to be too hurry to change. Actually, you are not easy to sell low quality players of level 1 or 2. There are many exciting fast trainers in Level 1 teams. I will love to keep them longer especially those performs well. Some of them will be good players for many seasons even if there is no special intensive training.