eusabius
There are currently no details of how the DWP intend to migrate ESA claimants to Universal Credit, so it's not possible to say what delays in payments that people should expect, however, UC is paid monthly rather than every the two weeks as ESA(IR) is so I think you should expect some gap in payments.
A reassessment will not result in an early transfer to UC, this will only happen if you need to make a new claim for another benefit that comes under the UC umbrella, Housing Benefit for example.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems