Have a situation where US residents / CA citizens (currently with green cards) are moving back to Canada and handing in their green cards. Doing so will benefit them on an extremely large taxable gain on a pre-IPO concentrated stock position. However, they have ~$500k in retirement assets – half in 401(k), half in a SEP IRA. They are in their mid-30s and in the highest tax income brackets. Any insight into the best tax situation for them? My understanding is that transferring to Canadian retirement plans will incur a taxable event plus penalty on the US side.