I don't think you can amend your trademark under the circumstances you describe. You certainly cannot if a registration has issued. I think a new trademark application would be in order even if the two marks are pronounced identically.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
I agree with Mr. Doland, and wanted to add that if you begin using a different spelling you are in effect using a different mark (though they are phonetically the same). This could lead to abandonment and other issues if you stop using the old one. Filing and registration of the new mark would be the safest approach.
Yes. The registration must be for the identical mark to what you use, not some variant. So if you alter the mark you need a new registration to capture all marks similar to the one you currently use. While it is likely true that "YOU'RE GREAT" and "YOU R GREAT" and "UR GREAT" might be considered similar, whether their use on products of two companies would be likely to confuse consumers depends on many factors, not just the phonetic similarity of the name. For one it depends on the respective consumers (channels of trade). You should file a new application, but it will not be "entirely new" as it would claim association with the prior registration if application is properly done. Also, the USPTO will make a pseudomark finding and you would want it to be that the two marks are psuedonyms, as that enhances proof of association for purposes of showing likelihood of confusion should you need to enforce either against an infringer of the other.
For $275 via TEAS PLUS, it would be incredibly poor business practice to NOT get a second registration. So, YES you really need to do this.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
I completely agree with my colleagues here. I would just add that obviously pursuing a trademark will be a big investment over time, so I really hope you have done your homework here beyond just dealing with the USPTO filing.
Whenever you endeavor into investing in a trademark it is very important that you conduct the proper clearance due diligence upfront and before you submit an application to the USPTO. In the US, this means searching under both federal (USPTO) as well as common law because trademark rights stem from use in this country NOT registration. This means that acquiring a federal registration does not necessarily mean that you are not infringing on another's intellectual property. See the link below for a detailed explanation of the due diligence process and a guide on how to choose a strong trademark.
I suggest that you consult with a lawyer in private and discuss your objectives in more detail. You can start by calling around to several for a free phone consultation, get some insights then pick the best fit to work with.
DISCLAIMER: this is not intended to be specific legal advice and should not be relied upon as such. No attorney-client relationship is formed with the law firm of Natoli-Lapin, LLC on the basis of this posting.