First start anything, we need have clear ideas and we have to plan any step before. If we wanna migrate to a cloud instance, as I wrote in a blog post, Use your head.

Plan all
Before execute a migration from server/on premise to cloud instance, think about features you use on Atlassian Tool in your environment.
Make a list of functionalities and classify them, so you can understand if are:
- Standard (common feature between server and cloud). We can use in both instances;
- Specific for server or cloud. Here we have a problem

For this second point, check all addons/apps installed and check which is used. In my experience, I always migrate instances with a tons of addons but only 2-3 were really used :-)
For each addon/app check if exists in cloud environment
- If yes, no problem. Check how migrate the data from server version
- If no, ok. Set PanicMode = ON :-P
If addon doesn't exists in cloud, well, this is the workaround :
- check if exists another one in cloud environment and test it. If cover all needs, analyze how convert information from server addon to cloud addon

- if some needs are not covered, check if others addons can help us and test them
- If you don't find another addon, well I believe this is not possible: the number of cloud addon increases every day and all functionalities are covered

An Example
In MrAddon blog, I've described how create calculated fields both in Server/on premise and cloud environments.

So we have possible solution if we have to migrate a server instance with calculated fields.
Conclusion
This is the first step of a big process. So:
- Use your head and mainly commonsense .
- Take your time and remember: Rome wasn't built in a day.