(2 part post):
The first point to make is that a court would normally be unaware of your previous conviction, so it shouldn't affect your chances of acquittal. (The prosecution can make an application to the court to refer to previous convictions, under the 'propensity' rules, but it's unlikely that such an application would be allowed in your case).
If someone is convicted of ABH the magistrates work from a 'starting point' of a (probably fairly short) custodial sentence. They then consider the various factors which can either reduce the sentence to a non-custodial one or, alternatively, increase the length of custody. Jake-the-Peg provided some useful information about sentencing, here:
http://www.theanswerbank.co.uk/Law/Question348 219.html
If you think that 'the odds are against you', it might make more sense to change your plea to 'guilty'. (That obviously applies if you actually committed the offence. Regrettably, with the way the law works, it might still be the best option even if you're innocent but the evidence suggests otherwise). The earlier you do so, the more weight it's likely to carry with the magistrates.