Hello,
1) Since the originals of the mark sheets are not tampered with there is no grave risk as of now.
2) Nevertheless refrain from such unwise timepass. This can land you in serious trouble.
3) The soft copies you have sent to the company is unlikely to be referred to again as you are already employed. You need not take the drastic step of resigning now.
4) For your peace of mind of you are getting a better opportunity elsewhere start anew with setting the records right. There is no safe shortcuts to success