If you do not comply wit the terms and conditions of probation the State will file a Motion to Revoke your probation. You are entitled to a hearing to determine if you have violated probation. If you revoke you face 6 months to 2 years in the State Jail. State Jail time is served day for day. To answer your question, yes you can get less than 2 years, in fact it is rare to receive the maximum sentence.
I'm not licensed in TX, but in NC a defendant may elect to serve the active portion of his sentence while on probation. How long you would go to jail depends on the law in TX. In NC a defendant may only serve half their sentence on a DUI/DWI. I suggest you discuss the specific facts of your situation with the attorney who originally represented you on the DUI. He would be in the best position to answer these questions for you.
If your probation is revoked, you are looking at a minimum of six months of state jail and up to 2 years in state jail. The only way to have it reduced is for the prosecutor and judge would agree to sentence the case under 12.44(a) which means a felony punished as a misdemeanor, which is highly unlikely to happen.
You should also realize that if your probation is revoked, your privilege to drive will be revoked and you will have a final felony conviction on your record for consideration by those with whom you are applying for a job.
You need to talk with a local lawyer to get real insight in the likelihood of a reduction, and if that lawyer feels it is possible, then you need to hire that lawyer.
You should consult with an attorney first. If you want off probation the attorney can discuss the issue with the judge and negotiate a lower sentence.
This is intended as only general legal advice. Feel to visit my website, www.austincriminaldefenseattorney.com for my blog on crime and punishment in Texas You may also email me through my site.