The valuation of your ETH depends on the current conversion rate and not on the rate it had at the moment of investment (also for committment in the case of ICBM investors). Therefore, once you invest, your ETH will stay with the value of the current conversion rate. The rate is available on chain via token price oracle, it's updated every 10 minutes from the feed provided by CoinMarketCap. The conversion to EUR happens at the moment of investment.