Each and every company is different. The best advice is to ask. Genuine reasons for not being able to move include seats being blocked for C of G, reserved for crew rest and seats where an additional charge is levied. Unfortunately, some cabin crew can be rather abrupt, rude and totally incorrect in their explanation, but right or wrong you have to do as they say. If I am asked and I have time I will always check to see how many people can move from one section to another, the answer is generally that it is OK.
You can see the effect of some of these inaccurate on posters to this thread. For example, many aircraft have seating zones for C of G calculations. If you move within the zone, you will not affect the C of G. A well known operator has a standard loading plan. This involves certain rows being blocked depending on passenger numbers. Providing the blocked rows are not sat in, people can sit wherever they want. Other loading systems generate trim settings for take-off. With these you can typically go wherever you want after departure. Moving back to your assigned seat for landing is typically not required. Others require a certain (aft) trim setting to obtain minimum drag (to avoid a tech. stop), therefore moving forwards is not OK. On public transport aircraft there is a reasonable margin beyond the published limits to allow for the weight ranges that make up averages and the occasional minor mistakes. Not surprisingly these are not always published.