Not afraid. I would consider this extremely unlikely here.
Not because it's impossible for terms to be changed, or someone to get an unfair deal. But because private retirement funds have strong defenders, while the central government's right to redistribute funds for the common good is viewed skeptically.
In addition, the trend or common problem here is that things happen when a private party can propose or manipulate a public process that subtly benefits the private party's private interest. The scenario you describe seems likely to have the opposite effect.
It is possible instead that private retirement funds could be damaged in a way where some private party (some person or company) benefits at the expense of another private party, perhaps with government sanction. This happened already, in the case where many private company pensions were weakened. Some private investors benefited; many pension holders received far smaller benefits than were originally promised. As a result of these events, investors on this site usually seek to invest directly in corporate stocks, or mutual funds based on such stocks. The underlying companies are strong forces in this society, unlikely to be defeated in political battles. Based on the most recent 40 years of history, anyway.
It's more likely that government pensions would be weakened, not private retirement funds. Some forum members do worry about this. The federal Social Security system is the biggest example. Currently it is projected to reduce benefits 25% starting in 2035 or so. Many threads discuss whether this will happen.