Players retire
You get a message on whether a player is going to retire or not 3 weeks before the season update. You get this information via your mailbox. The players who have decided to retire at the end of the current season have an hourglass by their names on the squad page. The risk of a player retiring depends on his/her age. If he/she turns 30 at the season update the risk of retirement is 1 in 40 and the risk increases for each year. One slight exception are goalkeepers who generally go on for a season or so longer than others. Even younger players under the age of thirty can decide to retire but the risk is very small, only 1 in 5000, so you do not have to worry too much about it even though it could happen. Every time you try to sell a player he/she will consider if he wants to end his career in the current club. This means that the player won't end his football career at the end of the season but he doesn't want to change clubs. He will play in your club as long as you want him, but you won't be able to sell him. The player might still decide to retire in the future though, but it won't be related to whether you have tried to sell him or not.
Skill level update
When a player's skill level changes the form changes in the opposite way. That is if a player's skill level increases his/her form gets worse and vice versa. The change in form does not change a player's actual capacity only how well he/she uses it. There is also a normal form update at the season update which makes it difficult to know exactly how much the skill level has changed by just studying the change in form.
When it comes to skill it is important to realize that the skill level you see is not the player's exact skill but a value that has been rounded off to the nearest whole number. So if a player has 6 skillboxes his/her true skill is somewhere between 5,50 and 6,50. This means a player can lower/raise his/her skill level quite a lot without it showing in the number of skillboxes and also that only a slight change in skill level can affect the number of skillboxes.
A form notation is a recording of a player's form at a point in time. A form notation is made when a player joins the team, at the season update, whenever a match is played (whether or not a player participated in the match), and when a random event received in the team's mailbox impacts a player's form. (Previous to Feb. 1, 2008, a notation was made when a form training was completed, but this has been removed under the changes instituted that day.) Each form notation is used to compute the player's average form. So if a player had notations of 10, 12, 14, 12, 10 and 8, the player's average form would be 11. (10+12+14+12+10+8)/6=11
What decides if a player raises his/her skill level or not is the player's average form and his/her age. All players who have an average form above 10 over the whole season will raise their skill level and those with an average form below 10 will lower their skill level. The higher the average form the more the skill level will be raised and vice versa. Younger players tend to do better in the skill level updates and raise their skill level more as well as lower their skill level less than older players. Read more about this in the section on the player and his/her form.
For a player to get 100% increase in skill level they will need to have received 100% match experience during the season. If the player for example only has received 60% match experience then the player will increase his skill with only 60% out of what the player would have increased his skill if the match experience would have been 100%. If the player faces a skill reduction due to an average form below 10 form bars and for example only has 60% match experience then the player's skill reduction will be 40% (100%-60%=40%) higher than it would have been if the player had 100% match experience.