In addition to Linked Servers (including adhoc uses via OPENQUERY, OPENDATASOURCE and OPENROWSET) calling an external app using xp_cmdshell can be affected if the called apps cannot run in a 64-bit shell. If you're using SSIS on the server any data access drivers can become an issue as well of which upgrading to use a 64-bit Excel driver is a common need.
From the client-side though, in terms of remote clients connecting to the instance, the change to 64-bit should not be detectable.
edit: should not be
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