I have been playing TSN/SOM for almost 6 years now. Over that time I have received plenty of good advice. I think the most important advice is to make sure you build a team that fits your ballpark. However, I think that close to importance is to know when to make changes.
So I will list the times that I feel it is beneficial to make changes:
1 - When you know that player(s) on your team is ill-suited for your ballpark
2 - When your team is not matching up well with teams in your divisions and you have a shot at the playoffs.
3 - Your team has been struggling all season and you cannot figure out why.
Now to go into those in more depth.
Firstly:
If you are a novice, then it may be likely that players you thought were good, are not ideal for your park. However you should always make sure in advance. If you are uncertain, then ask another owner, either in the league or on the forums ... preferably someone you know and who has given you reliable feedback before.
The quickest way to a losing season is to cut struggling players that were good fits. At best you will find a player that is 80 to 90% of the same value as your original player, because of the salary cap hit. Since this is a game of chance & die rolls, the 10-20% drop in value obviously will decrease the number of successful die rolls for your team.
Using a 2007 example - if you're in a pitchers park with Carlos Pena, you can assume he will hit for a low average but hit a lot of HR (due to his HR Chances). If he struggles early, and you cut him, you are very unlikely to find another bat who has the same amount of natural power. Perhaps you will find someone with better AVG or OBP, but most people draft Pena for his SLG Power, and that is not easy to replace, especially when you lose 1-2m in salary.
So if you're confident that the players you have are well suited, or at least ok for your park - and they struggle - then I strongly advise patience patience & more patience. I have had plenty of stud bats & pitchers struggle for weeks, only to finally start getting good die rolls and eventually come close to what they were originally expected to do.
Sure you will get the occasional dud season - but in my honest opinion - the likelyhood of an elite player having bad die rolls the whole season is always going to be less than the likelyhood of the player you pick up to have not only fit in the team better and have good die rolls.
Secondly:
If your team is always struggling against your divisional rivals, and by that I don't mean 4-5 or 5-7 records, but stuff like 3-6 or 2-10, and it is getting close to the trade deadline, then making a trade or cutting a player to get a new player that is of better use against your division.
However here you need to make sure that the player is a good match for both your team & to be used against the division. If your opponent has 4 LH Pitchers - don't drop a bat to pick up another hitter who is better suited against RH Pitching. If your opponents play in pitchers parks & you are in Citizens Bank, don't cut a pitcher to pick up another pitcher suited only for Citizens Bank. You will need to find players that will not hurt you (they do not have to help - just not be a negative) at home, but can do more damage against your division than your current player(s).
Think of it this way. If you are at the 138 game mark and are sitting 5-6 games out of first BUT with a 70-68 or similar record, which is enough to contend for the WC, and your team all season has lost most of its series to the divisional teams, then chances are that you won't make the playoffs.
With 24 games left, thats probably 9,9 & 6 against your 3 division opponents. That means you've played 12-15 of the 24 games you will play against each divisional rival. If your record was 5-10, 5-10 and 7-11 against those teams, then you can expect to go 10-14 or 9-15 the rest of the way, and those 2-3 extra games you lose will likely cost you the playoffs.
Even if you only go .500 with the changes ... that could be the extra wins you need to secure the WC. That could be the difference between going 78-84 and going 84-78.
Btw even if you're not in the playoff hunt - making a change to improve your chances against your division will give you more enjoyment in the role of spoiler. Sometimes even making a change will force your rivals to counter with a change, and changes made in re-action are less effective than those made pro-actively.
Finally:
If your team is 10-15 games below .500 with less than 3 weeks to go, then you're probably not going to succeed. At that point making changes won't hurt your playoff chances - since they were non-existant - but only allow you to test out new players to see how they fit into your team.
Conclusion:
Above all else - KNOW when to make a change & KNOW if the change is beneficial. If you were confident of the team and how it was built at the beginning of the season and you're 16-26 after 2 weeks it is NOT the end of the world. COLD streaks can happen just as frequently as HOT streaks.
I have had teams at 50-31 go 37-44 the rest of the way, and I have had teams start at 16-26 that rebounded to finish 5-10 games above .500.
Knowing when to make a change is the most important thing an owner can know after knowing how to design a team for his park.