If an investment is already classified as a long term investment, is it legal to convert it to a short term investment to improve current ratio?

You cannot be opportinistic, it is actually fraud to distort the financial statements to present a position that is not factual. The way to convert a long term investment to a current asset is to cash in the investment and reinvest in a short term asset i.e. cash in long term notes and invest in an interest bearing deposit maturing within one year.

A long term asset is an asset that has a maturity date beyond one year, a short term asset is an asset that has a maturity date within one year