Can they change the weights? Yes. Will they? It depends.
are ETF's fixed from their inception to their de-listing?
It's actually not possible for weights to be fixed, since different assets have different returns. So the weights are constantly changing as long as the market is moving. Usually after a certain period or a substantial market move, fund managers would rebalance and bring the weights back to a certain target.
The target weights - what your question is really about - aren't necessarily the same as the initial weights, but often times they are. It depends on the objective of the ETF (which is stated in prospectus). In your example, if the manager drops the weight of the most volatile one, the returns of the ETF and the 5 stocks could be substantially different in the next period. This is not desirable when the ETFs objective is to track performance of those 5 stocks.
Most if not all ETFs are passively-managed. The managers don't get paid for active management. So they don't have incentive to adjust the weights if their funds are tracking the benchmarks just fine.