When you are covered by an HDHP for only part of a year, your HSA contribution limit is prorated. If you are covered in December, you can use what is called the "Last Month Rule" to maximize your contribution, but the catch is that you must remain HSA-eligible (covered by an HDHP) for all of the following year. In your case, you used the last month rule in 2018, but failed to remain eligible all the way through 2019. The consequences are two-fold:
Whatever you over-contributed past your prorated limit, you need to add back in to your income on your next tax return, so that you can pay tax on the money that you deducted in error.
There is an additional 10% penalty on that amount.
These amounts will be calculated when you do your 2019 tax return on Form 8889, Part III.
Your question here is about re-categorizing your February 2019 contribution to be considered a 2019 contribution instead of a 2018 contribution. Unfortunately, you generally need to choose which tax year a contribution will belong to when you make the contribution, and you need to inform your HSA bank of your decision at that time. The bank reports to the IRS how much you contributed for each tax year. Since you've already specified to the bank that your February 2019 contribution was for 2018, and the bank has already reported this to the IRS (which they do in April), I don't think you can now choose to call that a 2019 contribution.
In IRS Publication 969, in the section about the Last Month Rule, there is a discussion about the consequences of failing to meet the testing period and some examples of how it works.
If you had caught this before you had done your 2018 return, you could have removed the excess contribution before April 15 and not claimed the Last Month Rule for 2018, but unfortunately you didn't know in time that you were going to fail the testing period.
