I worked on the proposal of a similar engagement. I may not be able to provide in depth guidance but here are my 2 cents.
1. Need of migration - Its important that you or your customer knows the need for migration. Tableau is expensive than Spotfire but at the same time more versatile than Spotfire.
2. Maturity of existing Spotfire reports - Maturity relates to the stability of reports in terms providing reliable data as well as periodic refresh processes. If the maturity is high, you will save efforts of "data discovery". You will not need run around to identify the right sources of data and then start using them. This bit is already sorted and this is a big plus. You will also know the required access and existing limitations. You will also possibly know the queries involved in fetching data into the reports.
3. Report formatting - A user would not like to move away from mature set of reports. However if the move is unavoidable, user would like to have exactly same functionality as existing toolset in the new toolset. Reports should look exactly same or similar in Tableau. Tableau has ample formatting features but you might not be able to replicate everything. On the positive side, you dont have to worry about color schemes, pallettes but you might need to find alternate ways to format specifically you are using dashboards.
4. Scheduling - Tableau provides scheduling capabilities so you can mimic existing scheduling if any. You will save time here.
5. User access and data security - You will need to do some work here as Tableau's access levels are different than what Spotfire offers. If the data access security is implemented at the database level then you can re-use it. That saves time.
6. Change Management - Moving to a new tool is always challeneg for new users. You will have to manage this change very well. User training and post go-live support until all dust is settled is required.
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