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More usual phrases to use in that situation are "until now," "at this point," or "at this time". Using "till" will may the reader think 'til, which is much more conversational.
It is an ok phrase to be used, although there are many more words that would be preferable to it, such as 'before [date of change]' or 'preceding [date]'
I don't see any problems. Till now is a very old and proper English phrase from which until derived. Shouldn't be a problem.
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