Will the SIM switch to the China+ IMSI as soon as it enters China automatically or after some time?
The point of switch to the China+ IMSI also depends on the device. The "SIM Application Toolkit" will switch automatically as soon as the device notice, that the country/region is changed.
1NCE Shop

Buy the 1NCE IoT Lifetime Flat now!

Visit the 1NCE Shop and start connecting your IoT devices easily. Simply order your SIM cards, choose the desired type of SIM card and fill out all required forms. After the payment has been approved you get your cards within two to three business days.