UIUC PREP is not a good idea if you are targeting the more popular majors.
https://dgs.illinois.edu/pre-engineering indicates that to go from PREP to ME, you need to earn a 3.75 overall and technical college GPA to be eligible for a competitive admission review. To go from PREP to AE, you need to earn a 3.50 overall and technical college GPA.
If you are in Purdue First Year Engineering, you can choose any engineering major with a 3.2 technical and overall college GPA. Lower GPAs mean competitive admission to majors.
https://engineering.purdue.edu/ENE/Academics/FirstYear/T2M
https://engineering.purdue.edu/Engr/InfoFor/CurrentStudents/enrollment-policy
Were you directly admitted to your major at GT? It does look like GT allows one unrestricted change of major within your first two years’ worth of credits.
https://advising.gatech.edu/change-major-glance
If all of them are unaffordable, do you have any other options that are less expensive and where you can get into your major?