As long as the 403(b) is completely terminated and all assets distributed, there are no legal issues. There are, however, two ways in which employers get tripped up on this.
Some 403(b) providers (particularly annuity issuers) have restrictions on the investments which may prohibit distributions except at the participant's direction. This can in some instances make it impossible to distribute assets, and thus to get the plan completely terminated. In some instances but not all, you can get around this by distributing the annuity contracts rather than cash, but you really have to look at the contracts.
Some employers want to just move all the existing money to the 401(k) plan. You can't do that, because there is no provision for plan to plan transfers from a 403(b) to a 401(k). You can give participants a choice of taking the money in cash or rolling it over to a vehicle of their choice (which could include the new 401(k) plan), but you can't restrict their choices.
There is no required 12 month wait. The rules that prevent distributions from a 401(k) if you set up a new 401(k) too soon do not apply if the old plan is a 403(b).