The phrase "alter existing" is correct and usable in written English.
You can use it when discussing changes or modifications to something that is already in place or established.
Example: "The committee decided to alter existing policies to better meet the needs of the community."
Alternatives: "modify current" or "change established".
When a legislature enacts a new law it exercises its normative power to alter existing legal rights, duties, etc., or create new ones that did not exist before that legal power was exercised.
The commission plans to alter existing laws by curbing the carmakers' design copyright on spare parts.
No writer worth the name sets out to produce happy or unhappy endings, let alone seeks to alter existing literature to produce one or the other.
The chemical's manufacturer, Bayer, claimed the report, released on Wednesday, did not alter existing risk assessments and warned against "over-interpretation of the precautionary principle".
The judge, Paula Manderfield, said the petition did not reflect that the proposal would alter existing provisions in the state constitution related to equal protection and discrimination.
Some digital systems enable engineers not only to alter existing sounds but also to synthesize new sound effects or music, including full symphonic scores.
Hamersley was trying to save money by altering existing buildings and serving as his own general contractor -- a situation that created problems throughout the project.
And Obama has even suggested making these children easier to deport by altering existing laws that provide special protections and legal reprieve for children crossing the border from Central America.
On Friday morning John had put the finishing touches to a homemade timetable he and Foster had devised, which proposed altering existing services in the Lune Valley to give four services a day instead of just one.
Trade ministers are also squabbling over altering existing trade agreements to help poor countries, which they also promised to do by the end of 2002.Moreover, farm trade is not some peripheral issue: it is central to the whole round.
Tuthill, trying to deal with the burden of altering existing houses, including even reusing the door trim, could not help bringing up "my sincere doubts as to whether or not you thoroughly appreciate what the total final effect is to be".
