In fairness to the vendors, the original RFP did not include a specific request for replacement, but during the initial meetings with the vendors the subject certainly came up. It should have been clear to any vendor that they did not have the staffing nor the expertise to migrate this data to the new platform. For users, then, the first word of caution is to make sure you include a request for conversion help, and possibly old equipment removal, in your RFP.
The next step is to understand how to make the conversion. You simply can't copy 30TBs of data from volume A to volume B. Your going to need some sort of plan to do that. This is often going to require an off-host tool that does a back-end migration, or a server-based tool that essentially replicates the data to the new storage system in the background. Either way, it is going to take a long time.
The back-end tools typically live within the switch infrastructure and perform a off-host copy of the data. In most cases, these will provide the fastest conversion. Several storage vendors have these as part of their internal software offering, others need to point you to a third party. Using file system tools or host-based migration tools may be a little slower and use server resources, but may be less expensive.
In either case, if your new system has thin provisioning as a key feature, then you want to make sure your migration software or your thin provisioning system can perform a thin migration. Many of the block-based copy tools will also copy empty blocks as well. If the copy tool or your new storage system doesn't know how to handle this data, then you're going to end up neutralizing this key feature, making your thinly provisioned volume fat. Having a thin aware file system or storage system should now be a key consideration when selecting a new storage platform.