I guess you are not using SSIS Package Configurations to store Connection Manager info. If you are you can simply change the config files and the code will not need to change.

If not, then you can try selecting this information from the package XML:

SELECT name

FROM msdb.dbo.sysdtspackages90

WHERE CAST(CAST([packagedata] AS VARBINARY(MAX)) AS VARCHAR(MAX))

LIKE 'servername\instancename';