Which part is broken ?
Usually this is fairly simple to debug and fix so that it's working perfectly.
Did you check that all your calculations are intact and not got disturbed like 
2 of 2 people found this helpful
One of the trickiest parts of this, for me, is if field names have been aliased. Tableau doesn't provide any obvious designation for aliased fields. So if, for instance, you're getting exclamation points in your calculated fields when you replace the data source, check the calculations to see if you get any messages that fields do not exist in data source. If that's the case, switch over to your original data source, find the field you need, right-click and select Describe, then see if there's a different name in the original data source. If so, rename the field in your new data source too. It's a bit annoying that Tableau doesn't take care of this automatically, but it doesn't.
The other trick is that if you're having problems with colors or formatting changing, you need to connect your new data source and in addition to the steps you mentioned, click on your sheet sorter view, then right-click and refresh all views. This pulls your formatting into memory so it's available to the new data source as well.
All other comments above are also useful.
Mostly the number formats are changed.