There are no simple answers.
Yes and No. I would tend to say 'no' but it depends on your current setup. Vaguely what I would watch for :
Is there a dynamic routing protocol that has learned the /28 ?
Is there any firewall rules based on the network /28 ? You will need to change the subnet to /26 also in the FW rules
Is there any devices configured with static IPs ? ( You will need to change the network mask and/or the default gateway on the devices )
Is there any DHCP pool related to that /28 ?
I'm pretty sure that the DHCP hosts on that subnet will need to renew their leases to obtain the updated network information ( net mask , default gateway )