"modified letters" is correct and usable in written English.
It refers to something that has been changed from its original form, such as a letter that has been adapted to fit a particular situation. For example, "When writing to the CEO, I used modified letters to make sure the tone was corporate and professional."
The final contact was made after an additional two weeks by a third modified letter and survey accompanied with reminder phone calls to non-respondents.
The sound values of modified Latin letters can often be derived from those of the original letters.
When the Cherokee Nation first purchased a printing press, some of Sequoyah's written characters were replaced by analogues from the Roman (Latin) alphabet or were created out of modified Roman letters for purposes of clarity, design, and aesthetics.
Additionally, to have a more precise idea of the timing of these effects, we modified the letter display, allowing us to monitor attentional allocation with a higher temporal resolution.
Two months after the initial mailing, a second questionnaire, modified cover letter, and pre-paid return envelope were mailed to initial survey nonrespondents.
The survey and invitation letter were sent to these volunteers and their suggestions were integrated into the survey and letter and the modified survey and letter were again sent out for review to these volunteers.
Consultants and referring doctors need to review, and modify their letter writing practices.
[C11.] The Pros and Cons of Upgrades A modified Dear John letter about upgrades on Continental Airlines draws both pros and cons.
A letter sent in August by a group of eight scientists claiming that "the lawmakers of the US Congress have modified, both in letter and spirit, the implementation of [the] agreement," prompted Mr Singh's reassurance to Parliament.
Mr. Craver said that many charities had modified their fund-raising letters to take account of recent events and that they had been very careful about putting their names and return addresses on very clearly.
