Create a new singleline text filed and copy data from your existing filed to new filed.
Create a on demand workflow or console app,or xrm toolbox bulk update tool to update all records.
now delete the old filed and create a multi line text filed. You can use the same schema name.
There will not be any other impacts on existing customization.
Now copy the data from your dummy filed to the text field.
Deployment
1) Create a new solution to add the dummy filed to the entity and do a managed deployment to the solution.
2) Perform the Data copy from the existing filed to the dummy filed
3) Create a new clone of the solution in the develop environment and remove the filed dependency from all forms, views etc. If the filed is existing in multiple solutions then it must removed from all solutions. Remove the filed from all cloned solutions
4) new deploy all solutions to the target environment as managed
5) Uninstall all old dependent solutions
6) This will remove the filed from the target environment
7) Now Increase the version number of the solution in development environment and create a new filed as multi line text and restore all removed customization
8) deploy the solution to the target environment
9) perform a data copy from dummy to the new filed
10) Uninstall the dummy filed solution from the target environment
