void removeExplicitThisParameter() {
		if (!allowCustomStorage &&
			CompilerSpec.CALLING_CONVENTION_thiscall.equals(callingConventionName)) {
			int thisIndex = findExplicitThisParameter();
			if (thisIndex >= 0) {
				parameters.remove(thisIndex); // remove explicit 'this' parameter
			}
		}
	}
Switch to custom storage and perform required transformations
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/function/editor/FunctionData.java