Had a call from Vodafone earlier today offering me the $39 open term plan for $29, and also knocking $10 off my home phone/broadband Vodafone package for having an on account mobile plan. I currently use prepay just with the $19 monthly pack so would work out the same monthly spend with the discount on my broadband package.

To summarise, with the $19 prepay plan I get:
Unlimited texts
100 minutes
500mb data

With the $29 open term on account deal it would be:
Unlimited texts
300 minutes
1.25gb data
Plus bonus: Extra 500mb per month for 12 months, and 1 NZ best mate (1000 mins, 1000 texts to any NZ number).

The on account deal looks good given my overall spend would stay the same. But I've only ever used prepay and feel a little uneasy about switching to a contract. I'm also a pretty light user. Only people I call tend to be my partner or parents and the 100 minutes usually tends to be enough for me (in fact I still have 97 minutes left this month and it renews on the 28th). The data is tempting though because I usually seem to use up the 500mb well before the end of the month, sometimes even in half the month. With the 300 minutes and already unlimited texts I probably wouldn't need the best mate anyway.

Is it worth the switch? Although they say "open term" could they still screw me in some way if I want to get out of it and go back to prepay?

As an aside, the person also emailed me the details so I didn't need to remember it all from the call and her email is at salmat rather than vodafone, who are Salmat?