Can I replace the IOS on a WS-C3550-48-SMI with one from a WS-C3550-48-EMI and then have an EMI switch rather than an SMI switch? Are there any drawbacks to doing this? Thanks.
There are no drawbacks to doing it and yes you can do it - so long as you have the right service contract with Cisco.
The image you want for EMI is IP Services (not IP Base).
See the Q/A here for SMI/EMI differences...