It's really hard to suppress chromosomal development, because if you don't have either XX or XY, you're most likely sterile. However, you can make it so all humans are always born with the same chromosome. You can choose XY, by injecting fathers with a spermicide that specifically targets X sperms; and vice versa you can force all humans to be born XX similarly by suppressing Y sperms. A bloodier, but much cheaper way to do this is to simply abort or kill all fetuses/babies with the "defective" chromosomes. There are advantages and disadvantages of whether you choose XX or XY. If the chosen chromosome is to be XY, it will be easier to revert to mixed chromosome culture by stopping treatments, but if you choose XX, there will be no turning back once the last XY died. So in the long run, maintaining XX-only civilisation is easier.
You can use hormonal manipulation to suppresses the hormones that causes the development of primary and secondary sexual characteristic. According to Wikipedia, development of primary sexual characteristic is controlled by hormones:
In the first weeks of life, a fetus has no anatomic or hormonal sex, and only a karyotype distinguishes male from female.
Specific genes induce gonadal differences, which produce hormonal differences, which cause anatomic differences, leading
to psychological and behavioral differences, some of which are innate and some induced by the social environment.
If your chosen chromosome is XY, you would want to decide whether you want to mess with the TDF/SRY gene. If you suppress TDF/SRY before gonodal differentiation, you'll trigger development of female sex organs, so all children will be genetically XY/male but physiologically female and have female reproductive organs, so those wanting to be male will need operations. If you don't suppress TDF/SRY, all children will be essentially male and those that chooses to be female will need operation.
Male-to-female surgery seems to be more expensive than female-to-male surgery, according to CostHelper at The Philadelphia Center for Transgender Surgery:
basic male-to-female surgery ... would cost a little over
\$30,000 while for basic female-to-male surgery ... would cost
between \$12,000 and \$25,000 depending on the type of genital surgery chosen.
For the cost-conscious governments, it seems that it'll be cheaper and preferable to have physiologically female children where half of the population will receive female-to-male gender reassignment surgery.
If you want to require medical intervention for both genders rather than just one, then you'll have to find a way you suppress the development of both male and female reproductive organs. In XY only population with TDF suppressed and in XX only population, you need to suppress the hormones that triggers vaginal development. This is to keep both pre-penis and pre-vagina cells undeveloped. In adulthood, you'll either artificially trigger pre-penis cells to start developing or undo the suppression of the hormones that triggers vaginal development. It's hard to know whether it would've been possible to trigger the development of primary sex organs at such a late age. Once the primary sex organ develops, the secondary sex organs should follow suit as they depend on the hormones produced by the primary sex organs.
Another complication for XY only population is that, since the mothers also have XY gene, then half of her egg will contain Y egg. That may cause two things, either reduced fertility because Y egg is either not viable or can only mate with X sperm. I don't know what would've happened with those born to the YY chromosome, they'll probably be non-viable. Also, if you want to maintain XY only population, you'll still have to suppress/abort all XX conception. This leads to a population with at best half the fertility of mixed chromosomes population. No such issues exists with XX only populations.
Note: I'm neither an expert in human reproductive system nor biology, so even though I try to base my answer on the sciences as I understand it, at least half of the things I said here is probably scientifically wrong.