that T-shirts logo is well known locally for what it is, so that part of it wont be misunderstood as much around here.
That may be the case, but... should you only be marketing to those for whom your logo is well known? or would it be better if the design is clear and understandable to people you haven't yet reached with your marketing?
And besides, simply changing the design to read "T-shirts" rather than "I-shirts" wouldn't be a big enough change to the design that it would somehow completely change the look and feel of your identity, and as a result, somehow cause you to lose business. On the contrary changing the design for the better would only help your business. I've always thought it was silly whenever a customer came in and refuses to change a poor design because
they think it is "established". You don't want to "establish" a logo that is confusing, or worst yet a logo that looks like a mistake was made, especially when you are in the business of handling other peoples brands or advertisements.