IRS Notice 2004-50 Q&A 20 and IRS Notice 2008-59 Q&A 40 provide the answer to your question.
Only qualified medical expenses incurred after the establishment of the HSA account (generally the date of the first contribution to the HSA account). However, if the new HSA was funded with a rollover from the old HSA account, the establishment date of the old HSA account transfers to the new HSA account.
From your description it appears that the establishment date of the new HSA account would be a date in 2017, so no medical expenses incurred before that date in 2017 would be qualified medical expenses with respect the new HSA account.
https://www.irs.gov/irb/2004-33_IRB/ar08.htmlhttps://www.irs.gov/irb/2008-29_IRB/ar11.html
