If a same company and same manager run contests, rules can be not too different while there is always possible chances if the company and manager see needs of changing contest rules for better effects.
If it a same company but contests hosted by different managers, you will have higher chances of many different terms. People as possible participants must read terms very carefully before joining if they don't want to miss important information. They can be disqualified by ignoring contest rules and this possibly causes initial fund loss by lack of carefulness for Reading before Joining.
Yes, they may not change the rules but if they think it is necessary to change, they will do that but they will announce the new rules to their members. But in other casinos, the rules will be different so we must check it before we participate. We need to be aware of the rules so we will not have a problem after we participate the contests.
Sometimes, they can introduce a modified rules that will not easy to fill the requirements. But if we understand the rules and we already ask to their support system regarding the modified rules, we will not break the rules. It is why those who want to participate in the contests should read the rules so they will not if the requirements is easy or difficult to fill.