Advice please on how and when to change training plans for Ironman training

[Kiwi]

Hi all - advice please - I have been training up for 2 different Half IM events in Dec before recently making the decision to do the Full IM in Taupo in early March 2019. I have done Half IM Base and now completing the Build phase - plan was to move to Half IM Specialty phase next week to tune up for Half IM’s …Now I have decided to do the Full IM in March - do I carry on with Specialty phase for Dec races or should I move into the Base phase for a full IM plan and progress through the full IM plan (6 weeks Base, 8 weeks Build then 8 weeks Specialty) …21 weeks until Full! Not sure how moving to the Base plan for Full IM will impact on Halfs in Dec