Just sanitize the keg as is, but do yourself a favor and remove the liquid tube. Mix up your posts or QD's and that keg will empty everywhere. trust me, you'll only do it once. Use fermcap to keep the krausen down. When it comes time to transfer, replace the liquid dip tube and pressurize to 5 psi. Use a liquid to liquid jumper and you can transfer to a clean keg.
Use swivel connectors and take one QD off the jumper so you can bleed off the first pint of sediment. Then pull the liquid out QD on the original keg so you can stop the flow. Hook up the second QD again to the jumper. Transfer as normal. Easy peasy.