I would guess that an English version of this program would be just as tedious to write as it would be to manually correct the documents.
Programmers often fall prey to this kind of thinking. We've all done it - faced with an odious manual solution, we write a program or script that ends up taking as long or longer to get right than it would have to get a shovel and do it manually.
Break the problem into parts. Start with a grammar for English that can distinguish the verbs from all the other words in every case.
Get a dictionary of verb tenses for the verbs you'll need.
Walk the abstract syntax tree and substitute the past tense verbs for their present tense counterparts.
You don't say what computer language you're comfortable with. If it's Java, I'd recommend looking at ANTLR. It's a wonderful parser/generator for Java.
When I say "write a program", I'm thinking that you'd start with something like this.
Recognize that getting a significant portion of the job in one go might be worth it. But no matter what, you'll have proof read each and every one to make sure that the code did the right thing or risk a public gaffe.
There's another possibility: crowd source it. Give one document each to 30+ people and ask them to do the laborious job for one document. Better yet, break those into parts and use more people. Parallelism is your friend here.