What are the advantages of HFSS 11 over HFSS 10?

Faster, lower memory requirement

becoz of reduced number of tetrahedras for the same model

You also get to learn a new/modified interface. Keep a close eye on the reports as is is easy to screw that up with the changes. There are good features too.

Has the interface changed that much? Hopefully for the better!

Are there any issues with the migration of projects?

New in HFSS v11



New higher-order hierarchical basis functions combined with an iterative solver provides accurate fields, smaller meshes, and efficient solutions for large multi-wavelength structures.
Enhanced port solver: HFSS can simulate lower frequencies than before and will have a lower noise floor.
Floquet Ports: these periodic ports enable designers of phased-array antennas and frequency-selective surfaces to obtain separate fields into modes for better understanding and examine phase information for each mode.
Optimetrics? now offers genetic algorithm
New Auto-assign capability for terminals
Complex geometries solve 2-5X faster using half the memory with the new v11 fault tolerant, high-quality finite element mesher
Expanded Distributed Solve Option
Automatic Distributed Solve of discrete and interpolating frequency sweeps
Now available for parametric sweeps, optimization, sensitivity analysis, and statistical analysis
Improved Dynamic Links SIwave-HFSS and HFSS-HFSS
Link is now based on both E- and H-field; greater accuracy in case of enforced field in HFSS-HFSS link
New report editor for better usability
Feature-rich 2D plots