There would not be any tax implications of a foreign entity acquiring a US corporation nor would there typically be any impact upon the debt converting to equity. There would be interest income on the debt, which would be taxable income and if the entity later paid dividends to the equity holders those dividends would be taxable. Taxation of any capital gain upon disposition of the investment would depend on a number of factors and would require an additional discussion at the time of disposition.

Just to add a point. Generally, dividends and interest paid by US corporation to non-US entities would be subject to a 30% US withholding tax.