An interesting topic, personally I'd be all for new ships (new Nepthys model please?)
Baconsoda however I agree with, with the exception that there is precedent for the introduction of new aircraft in the life cycle. Take the F22 Raptor (even with its problems) for example.
Secondly, I would add the example of the F/A-18 Hornet. It was introduced in 1983 and now is known as the F/A-18 Super Hornet, sooo
From this perspective the logical thing to do would be to showcase the guardian as a "superhornet" or upgraded defender, where as maybe give liberty an entirely new fighter to represent the introduction of new systems and what not.
So therefore, keep the current mark IIs, give every house faction a new second generation VHF
EDIT: I must correct myself, the Super Hornet is a complete redesign of the hornet and is apparently bigger than the original Hornet (according to wikipedia)