The disclosure comes amid increasing numbers of applications for places at both GM primary and secondary schools - and less than a month after the Government pledged to make it easier for them to introduce selection.
The survey of more than 1,000 opted-out schools showed applications were up in half of GM secondaries and a third of GM primaries.
Most of the schools which had changed their admissions policies - 56 per cent - had done so to give siblings of current or former pupils higher priority.
Nearly a quarter, however, had increased the proportion of pupils selected by ability or aptitude.
A significant proportion of the survey sample - almost 20 per cent of the secondary schools - already select pupils by ability.
Last month's White Paper promised grant-maintained schools they would be able to select 50 per cent of their pupils without having to publish statutory proposals if the Conservatives win the general election.
Local authority specialist and comprehensive schools would be able to select 30 per cent and 20 per cent of their pupils respectively.
The White Paper also pledged to make it easier for the GM sector to set up sixth forms and nurseries.
The survey, conducted by Research International for the Grant-Maintained Schools Foundation, revealed that 16 per cent of schools had considered or applied for a nursery, 11 per cent for a sixth form and 6 per cent to change the age range.
It showed that a third of GM schools had increased staffing levels by more than 10 per cent and that one in 10 had taken on more than 25 per cent extra staff.
More than 40 per cent of opted-out schools said they had lowered their pupil-teacher ratios - but nearly 18 per cent had increased them.
And 76 per cent reported that since going GM there had been an improvement in staff morale, with 27 per cent reporting great improvement.
The survey had a response rate of 68 per cent of all GM schools in England and Wales (734 schools out of the 1,074-strong sector).
More than eight out of 10 schools said they had increased spending on books and teaching equipment and a similar number said that more had been spent on furniture, maintenance and decoration.
Nearly two-thirds said they had improved their position in the performance league tables and 62 per cent now offered new subjects.
Substantial numbers of schools who no longer used their former local authority for catering, cleaning and payroll reported a better service.
And Sir Robert Balchin, chairman of the Grant-Maintained Schools Foundation, said: "On every single measure the grant-maintained sector proves again and again that schools can outperform LEA bureaucracies.
"It is time for the dead hand of LEA bureaucracy to be removed from state school management once and for all."