First of all, I congratulate you for your work. Of course, you can adjust your hypotheses according to the survey. However, it is more difficult than doing a survey from scratch. You cannot go beyond the purpose, place and time frame in which the survey was conducted. You cannot go far beyond the results of the evaluation of this survey. In addition, to propose a new hypothesis, you must show information that has not been revealed or sufficiently illuminated. Or there have been new developments and you can propose new hypotheses and try to reach the results by making improvements with this survey. However, since it is a survey that has already been obtained and used, your job is not easy. So you have to make your work very careful and meticulous. Good luck.

Good effort!

Once you adjust the question, you affect the validity of the meager, immediately. Also, your hypothesis should not depend on what scale is available. If your hypothesis is nov, then pursue a novel agenda (exploratory study) where you can create your own survey and begin from there. If you decide to use the ready made scale, try testing for invariance, after the modification.

Good luck

If you decide to modify the questionnaire (major changes) , you are now providing a new approach which is an extension or addendum however, if these changes are small, they usually do not make significant difference).

In term your changes are major, you need to prove Why would you do these changes? and in which base you have done it, otherwise every researcher can amend and publish their results without proves the validity of their instrument. However, another way to look at this issue is if you manage successfully to upgrade or amend any instrument (questionnaire) and it has proven its validity, it considers a novel contribution to the knowledge in your research area.